A North Korean soldier stands guard along the bank of Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, May 1, 2014. Picture taken May 1, 2014. REUTERS/Jacky Chen (NORTH KOREA - Tags: MILITARY POLITICS TPX IMAGES OF THE DAY) 

A North Korean soldier kicks a pole along the bank of Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, February 4, 2014. Picture taken February 4, 2014. REUTERS/Jacky Chen (NORTH KOREA - Tags: MILITARY POLITICS) 

A North Korean soldier gestures as he holds a sickle in a field on the Hwanggumpyong Island, located in the middle of the Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, January 23, 2014. Picture taken January 23. REUTERS/Jacky Chen (NORTH KOREA - Tags: MILITARY TPX IMAGES OF THE DAY) 

A North Korean soldier gestures as he holds a goat in a field on the Hwanggumpyong Island, located in the middle of the Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, January 23, 2014. Picture taken January 23, 2014. REUTERS/Jacky Chen (NORTH KOREA - Tags: MILITARY ANIMALS TPX IMAGES OF THE DAY) 

North Korean girls carry firewood on their backs as they walk on the banks of Yalu River, some 100 kilometers (62 miles) from the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, December 16, 2013. U.S. Secretary of State John Kerry described North Korea's Kim Jong Un as reckless and insecure after the execution of the leader's powerful uncle, and said Kim's actions underscored a need for a unified stand against Pyongyang's nuclear program. Picture taken December 16, 2013. REUTERS/Jacky Chen (NORTH KOREA - Tags: POLITICS SOCIETY TPX IMAGES OF THE DAY) 

A North Korean waves his fist on the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, December 7, 2013. REUTERS/Jacky Chen 

A North Korean woman sits on a trolley along the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, October 3, 2013. REUTERS/Jacky Chen 

North Korean soldiers smoke on the banks of Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, May 20, 2013. REUTERS/Jacky Chen 

North Korean children wave during their trip on the Yalu River in Sinuiju, opposite the Chinese border city of Dandong, May 15, 2013. REUTERS/Jacky Chen 

North Korean soldiers are seen through fences as they work with shovels on Hwanggumpyong Island, located in the middle of the Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, April 13, 2013.
REUTERS/Jacky Chen 

North Korean soldiers (top) bend to dodge the Friendship Bridge connecting the Chinese city of Dandong with Sinuiju in North Korea as people take a ferry on the 101st birthday of North Korea's founder Kim Il-sung, on the Yalu River near Sinuiju, April 15, 2013. REUTERS/Jacky Chen 

Female North Korean soldiers patrol along the banks of Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, April 11, 2013. REUTERS/Jacky Chen 

A North Korean soldier looks out of the window of a guard tower, on the banks of Yalu River, about 100 km (62 miles) from the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, April 16, 2013. REUTERS/Jacky Chen 

A North Korean man blocks his face with his hand from being photographed as he and other residents take a ferry in Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, April 11, 2013. REUTERS/Jacky Chen 

A North Korean soldier washes his socks as his comrade washes his hands at the banks of Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, April 8, 2013. REUTERS/Jacky Chen 

A North Korean soldier holds a badminton racket on the banks of Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, May 20, 2011. REUTERS/Jacky Chen 

A North Korean soldier kicks a goat on the banks of the Yalu River near the North Korean town of Sinuiju, July 5, 2009. REUTERS/Jacky Chen

North Korean soldiers practice throwing grenades on the banks of Yalu River in the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, March 6, 2013. REUTERS/Jacky Chen 

North Korean soldiers joke while guarding the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong April 5, 2010. REUTERS/Jacky Chen 

Children throw stones into Yalu River, at the river banks, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, March 30, 2013. REUTERS/Jacky Chen 

A North Korean soldier guards an army installation on the banks of the Yalu River at the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, October 10, 2006. REUTERS/Reinhard Krause

A North Korean soldier uses a pair of binoculars to monitor a Chinese tourist boat on the bank of Hwanggumpyong island located in the middle of the Yalu River, near the North Korean town of Sinuiju and the Chinese border city of Dandong, April 30, 2012. REUTERS/Jacky Chen 

A North Korean soldier waves to a Chinese tourist boat on the banks of Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, January 7, 2011. REUTERS/Stringer 

A North Korean soldier gestures toward a Chinese tourist boat after monitoring with a pair of binoculars on the bank of Hwanggumpyong island located in the middle of the Yalu River, near the North Korean town of Sinuiju and the Chinese border city of Dandong, April 30, 2012. REUTERS/Jacky Chen 

A North Korean soldier looks into an entrance of a military base on the banks of Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, March 1, 2012. REUTERS/Jacky Chen 

A North Korean woman and soldiers look at a Chinese tour boat from the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, May 28, 2009. REUTERS/Jason Lee

A North Korean soldier works on the banks of the Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong November 24, 2010. REUTERS/Stringer 

North Koreans work on the roof of a house along the banks of the Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, November 23, 2010. REUTERS/Stringer 

A female North Korean soldier stands guard on the banks of the Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, November 23, 2010. REUTERS/Stringer 

North Korean soldiers unload foodstuffs on the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, March 27, 2010. REUTERS/Jacky Chen 

A North Korean gets a shave on the bank of Yalu River near the North Korean town of Sinuiju August 5, 2009. REUTERS/Jacky Chen 

North Korean soldiers react to being photographed while exercising at an army installation on the banks of the Yalu River at the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, October 10, 2006. REUTERS/Reinhard Krause

North Korean soldiers guard the bank of Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, July 3, 2009. REUTERS/Jacky Chen 

North Korean soldiers play volleyball on the bank of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, June 13, 2009. REUTERS/Jacky Chen

North Koreans watch from the banks of the Yalu River at the North Korean town of Sinuiju as a Chinese tour boat passes by October 10, 2006. REUTERS/Reinhard Krause

A North Korean soldier looks out with binoculars at a outpost along the banks of Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, March 29, 2013. REUTERS/Jacky Chen 

A North Korean soldier urinates on the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, March 27, 2010. REUTERS/Jacky Chen

North Koreans wash their laundry as others skate on the ice at the Yalu river at the border to China near the North Korean city of Hyesan November 30, 2008. REUTERS/Reinhard Krause

North Korean children wave to people on a Chinese tourist boat on the banks of Yalu River near the Chongsong county of North Korea, opposite the Chinese border city of Dandong, May 8, 2011. REUTERS/Jacky Chen 

A North Korean soldier exercises near an army installation on the bank of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border town of Dandong, October 19, 2006. REUTERS/Nir Elias

A North Korean boy skates on the ice of the Yalu River at the border to China near the North Korean city of Hyesan December 1, 2008. REUTERS/Reinhard Krause

North Korean women hold umbrellas near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong March 31, 2010. REUTERS/Jacky Chen

A North Korean man comes down a ladder in front of a disused factory along the banks of the Yalu River near the North Korean town of Qing Cheng, north of the Chinese border city of Dandong September 12, 2008. REUTERS/David Gray

North Korean border guards joke around while on duty at the Yalu river at the border to China near the North Korean city of Hyesan November 30, 2008. REUTERS/Reinhard Krause

A North Korean soldier reads at an army installation on the banks of the Yalu River at the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, October 10, 2006. REUTERS/Reinhard Krause

A woman carrying a baby on her back rides on a motorbike with a North Korean soldier along the banks of the Yalu River near the North Korean town of Qing Cheng, located around 50km (31 miles) north of the Chinese border city of Dandong, September 12, 2008. REUTERS/David Gray

A North Korean soldier patrols on the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, May 5, 2014. Picture taken May 5, 2014. REUTERS/Jacky Chen (NORTH KOREA - Tags: MILITARY SOCIETY TPX IMAGES OF THE DAY) 

North Korean soldiers smoke cigarettes as they stand on the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, May 4, 2014. Picture taken May 4, 2014. REUTERS/Jacky Chen (NORTH KOREA - Tags: MILITARY SOCIETY) 

A North Korean soldier sits inside a sentry post as he guards on the banks of the Yalu River near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, May 4, 2014. Picture taken May 4, 2014. REUTERS/Jacky Chen (NORTH KOREA - Tags: MILITARY SOCIETY TPX IMAGES OF THE DAY)