A North Korean defector and his children stand in front of a barbed wire fence near the demilitarised zone separating the two Koreas, in Paju, February 12, 2021, on the occasion of Seolnal, the Korean Lunar New Year's day. REUTERS/Kim Hong-Ji

A North Korean refugee cries after having a memorial service for his North Korean ancestors in front of a barbed wire fence near the demilitarised zone separating the two Koreas, in Paju, February 12, 2021, on the occasion of Seolnal, the Korean Lunar New Year's day. REUTERS/Kim Hong-Ji

A North Korean refugee leaves the altar for a memorial service for his North Korean ancestors, near the demilitarised zone separating the two Koreas, in Paju, February 12, 2021, on the occasion of Seolnal, the Korean Lunar New Year's day. REUTERS/Kim Hong-Ji

A couple looks toward the North on an observatory near the demilitarised zone separating the two Koreas, in Paju, February 12, 2021, on the occasion of Seolnal, the Korean Lunar New Year's day. REUTERS/Kim Hong-Ji

Hyeon Seong-taek, a North Korean refugee, looks toward the North as he sits next to an altar for a memorial service for his North Korean ancestors, in front of a barbed wire fence near the demilitarised zone separating the two Koreas, in Paju, February 12, 2021, on the occasion of Seolnal, the Korean Lunar New Year's day. REUTERS/Kim Hong-Ji

Relatives of North Korean refugees set up an altar for a memorial service for their North Korean ancestors, near the demilitarised zone separating the two Koreas, in Paju, February 12, 2021, on the occasion of Seolnal, the Korean Lunar New Year's day. REUTERS/Kim Hong-Ji

Representation of cryptocurrency is seen in front of a Kucoin logo in this illustration taken on February 9, 2021. REUTERS/Dado Ruvic/Illustration

Representations of cryptocurrency is seen in front of a Kucoin logo in this illustration taken on February 9, 2021. REUTERS/Dado Ruvic/Illustration

Representations of cryptocurrency is seen in front of a Kucoin logo in this illustration taken on February 9, 2021. REUTERS/Dado Ruvic/Illustration

Representations of cryptocurrency is seen in front of a Kucoin logo in this illustration taken on February 9, 2021. REUTERS/Dado Ruvic/Illustration

South Korean Unification Minister Lee In-young speaks during a press conference at the Seoul Foreign Correspondents' Club in Seoul, South Korea, February 3, 2021. Ahn Young-joon/ Pool via REUTERS

U.S. President Donald Trump meets with North Korean leader Kim Jong Un at the demilitarized zone separating the two Koreas, in Panmunjom, South Korea, June 30, 2019. Kevin Lamarque: "The much-anticipated meeting with North Korean Leader Kim Jong Un and President Donald Trump on the DMZ (Demilitarized Zone) ... was a perfect storm of the security services from three countries trying to keep the media from three countries at bay and their leaders safe. This photo came about after Trump crossed the DMZ demarcation line into the North Korean side to meet Kim. I think the photographer in motion trying to get his position adds a sense of urgency and stress to the moment." REUTERS/Kevin Lamarque/File Photo SEARCH "PHOTOGRAPHERS TRUMP" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY.

The facility is suspected of being a covert part of North Korea's nuclear weapons programme, possibly for enriching uranium fuel for weapons or producing components such as centrifuges.

South Korean Foreign Minister Kang Kyung-wha, speaking with his Chinese counterpart in Seoul, South Korea, November 26, 2020. Kim Min-hee/Pool via REUTERS

Le nombre de transfuges nord-coréens entrés en Corée du Sud.

Reuters Graphics

South Korean soldiers stand at the southern side of Panmunjom in the Demilitarized Zone, South Korea, Wednesday, November 4, 2020. Lee Jin-man/Pool via REUTERS

Kim Yo Jong, sister of North Korea's leader Kim Jong Un attends wreath laying ceremony at Ho Chi Minh Mausoleum in Hanoi, Vietnam March 2, 2019. REUTERS/Jorge Silva/Pool BEST QUALITY AVAILABLE

North Koreans march alongside a float celebrating the country’s socialist health system during a parade in Pyongyang, North Korea, September 9, 2018. Picture taken September 9, 2018. REUTERS/Josh Smith

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, and lawmakers get into the Office of the United Nations High Commissioner for Human Rights, with a document to appeal for a U.N. investigation on his brother’s death, in Seoul, South Korea, October 6, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, and lawmakers get into the Office of the United Nations High Commissioner for Human Rights, with a document to appeal for a U.N. investigation on his brother’s death, in Seoul, South Korea, October 6, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, and lawmakers get into the Office of the United Nations High Commissioner for Human Rights, with a document to appeal for a U.N. investigation on his brother’s death, in Seoul, South Korea, October 6, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, poses for photographs with a document to appeal for a U.N. investigation on his brother’s death, during a news conference in front of the Office of the United Nations High Commissioner for Human Rights, in Seoul, South Korea, October 6, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, and lawmakers arrive at the Office of the United Nations High Commissioner for Human Rights, in Seoul, South Korea, October 6, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, and lawmakers attend a news conference in front of the Office of the United Nations High Commissioner for Human Rights, in Seoul, South Korea, October 6, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, speaks during a news conference in front of the Office of the United Nations High Commissioner for Human Rights, in Seoul, South Korea, October 6, 2020. REUTERS/Kim Hong-Ji

Jo Kyeong-hyeon, who was originally from the North, holds a memorial service for his North Korean ancestors on the occasion of Chuseok, the Korean Thanksgiving Day, near the demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

North Korean defector Han Hong-geun (not pictured) and his family prepare for a memorial service for their North Korean ancestors on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

A woman stands behind a military fence on the occasion of Chuseok, the Korean Thanksgiving Day, near the demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

Jo Kyeong-hyeon, who was originally from the North, looks on as his family members prepare for a memorial service for their North Korean ancestors on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

Jo Kyeong-hyeon, who was originally from the North, looks on a memorial altar for their North Korean ancestors on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

Visitors look toward the north from the observation platform on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

North Korean defector Han Hong-geun and his family prepare for a memorial service for their North Korean ancestors on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

A North Korean defector Han Hong-geun (L) and his family make a deep bow during a memorial service for their North Korean ancestors on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

A North Korean defector Han Hong-geun (L) and his family prepare for a memorial service for their North Korean ancestors on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

Visitors tour Mangbaedan Altar which is the place for refugees or defectors to ease their homesickness, on the occasion of Chuseok, the Korean Thanksgiving Day, near the Demilitarised zone separating the two Koreas in Paju, South Korea, October 1, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, speaks during a news conference in Seoul, South Korea, September 29, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, arrives at a news conference in Seoul, South Korea, September 29, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, speaks during a news conference in Seoul, South Korea, September 29, 2020. REUTERS/Kim Hong-Ji

Lee Rae-jin, the older brother of a South Korean fisheries official shot dead by the North Korean military, reacts during a news conference in Seoul, South Korea, September 29, 2020. REUTERS/Kim Hong-Ji

North Korean soldiers keep watch toward the south as South Korean Unification Minister Lee In-young inspects (not pictured) the truce village of Panmunjom inside the demilitarized zone separating the two Koreas, South Korea, September 16, 2020. Korea Pool/Pool via REUTERS

South Korean Unification Minister Lee In-young inspects the Peace House at the truce village of Panmunjom inside the demilitarized zone separating the two Koreas, South Korea, September 16, 2020. Korea Pool/Pool via REUTERS

North Korean soldiers keep watch toward the south as South Korean Unification Minister Lee In-young inspects (not pictured) the truce village of Panmunjom inside the demilitarized zone separating the two Koreas, South Korea, September 16, 2020. Korea Pool/Pool via REUTERS

South Korean Unification Minister Lee In-young inspects the inter-Korean hotline at the truce village of Panmunjom inside the demilitarized zone separating the two Koreas, South Korea, September 16, 2020. Korea Pool/Pool via REUTERS

South Korean Unification Minister Lee In-young inspects the inter-Korean hotline at the truce village of Panmunjom inside the demilitarized zone separating the two Koreas, South Korea, September 16, 2020. Korea Pool/Pool via REUTERS

South Korean and U.S. Army soldiers stand guard at the truce village of Panmunjom inside the demilitarized zone separating the two Koreas, South Korea, September 16, 2020. Korea Pool/Pool via REUTERS

South Korean Unification Minister Lee In-young gets a briefing from a U.S. Army official at the truce village of Panmunjom inside the demilitarized zone separating the two Koreas, South Korea, September 16, 2020. Korea Pool/Pool via REUTERS

A submerged Han River park by flooded Han River is seen in Seoul, South Korea, August 6, 2020. REUTERS/Kim Hong-Ji

A highway that has been emptied by flooded Han River is seen in Seoul, South Korea, August 6, 2020. REUTERS/Kim Hong-Ji

Residents look on a submerged Han River park by flooded Han River in Seoul, South Korea, August 6, 2020. REUTERS/Kim Hong-Ji

A man takes photographs of flooded Han River in Seoul, South Korea, August 6, 2020. REUTERS/Kim Hong-Ji

Kwon Cha-soon poses for photographs at a makeshift shelter for flood sufferers at a gym in Ansung, South Korea, August 5, 2020. REUTERS/Daewoung Kim

A health worker sanitizes the floor of a makeshift shelter for flood sufferers at a gym in Ansung, South Korea, August 5, 2020. REUTERS/Daewoung Kim

A health worker sanitizes the floor of a makeshift shelter for flood sufferers at a gym in Ansung, South Korea, August 5, 2020. REUTERS/Daewoung Kim

General view shows a makeshift shelter for flood sufferers at a gym in Ansung, South Korea, August 5, 2020. REUTERS/Daewoung Kim

Kim Soo-goon uses his mobile phone at a makeshift shelter for flood sufferers at a gym in Ansung, South Korea, August 5, 2020. REUTERS/Daewoung Kim

U.S. Gen. Robert Abrams, commander of the United Nations Command, Combined Forces Command, and United States Forces Korea, delivers a speech during the commemorating ceremony for the U.N. Forces Participating Day in the Korean War at the border village of Panmunjom in Paju, South Korea July 27, 2020. Ahn Young-joon/Pool via REUTERS

U.S. Deputy Secretary of State Stephen Biegun, left, and Japanese Vice Foreign Minister Takeo Akiba, right, greet each other prior to their bilateral meeting at Iikura Guest House Thursday, July 9, 2020, in Tokyo, Japan. Eugene Hoshiko/Pool via REUTERS

U.S. Deputy Secretary of State Stephen Biegun, left, and Japanese Vice Foreign Minister Takeo Akiba, right, greet each other prior to their bilateral meeting at Iikura Guest House Thursday, July 9, 2020, in Tokyo, Japan. Eugene Hoshiko/Pool via REUTERS

U.S. Deputy Secretary of State Stephen Biegun, left, and Japanese Vice Foreign Minister Takeo Akiba, right, walk together prior to their bilateral meeting at Iikura Guest House Thursday, July 9, 2020, in Tokyo, Japan. Eugene Hoshiko/Pool via REUTERS