A young Central American migrant, moving in a caravan through Mexico, disembarks from a freight train as he carries his belongings on a railway track after stopping the train on a rail line, in Irapuato, Guanajuato state, Mexico April 15, 2018. REUTERS/Edgard Garrido 

Candles and flowers are placed at the site where, on April 7, a man drove a van into a group of people sitting outside a popular restaurant in the old city centre of Muenster, Germany April 8, 2018. REUTERS/Leon Kuegeler 

Flowers are seen on the site where on April 7 a man drove a van into a group of people sitting outside a popular restaurant in the old city centre of Muenster, Germany April 8, 2018. REUTERS/Wolfgang Rattay 

Central American migrants, moving in a caravan through Mexico toward the U.S. border, eat at a shelter set up for them by the Catholic church, in Puebla, Mexico April 6, 2018. REUTERS/Edgard Garrido 

A Central American migrant, part of a caravan moving through Mexico toward the U.S. border, holds cigarettes during a break on their route to Puebla, on a highway in Veracruz, Mexico April 6, 2018. REUTERS/Edgard Garrido 

Honduran migrants hold their national flag during a march along a street in the city as they take a pause from traveling in their caravan, during their journey to the U.S., in Matias Romero, Mexico April 3, 2018. REUTERS/Henry Romero 

A woman cries as she attends a gathering to protest the death of a street hawker in central Madrid, Spain, March 16, 2018. REUTERS/Susana Vera TPX IMAGES OF THE DAY 

A woman cries as she attends a gathering to protest the death of a street hawker in central Madrid, Spain, March 16, 2018. REUTERS/Susana Vera 

Migrants are silhouetted while resting next to the sea as a ferry sails for Italy, from the port of Patras, from where migrants try to smuggle themselves onto ferries, in Patras, Greece, March 7, 2018. REUTERS/Alkis Konstantinidis SEARCH "KONSTANTINIDIS PORT" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

A woman lights a candle as she prays for victims of the March 11, 2011 earthquake and tsunami disaster, at a park in Tokyo, Japan, March 11, 2018, to mark the seventh-year anniversary of the disaster that killed thousands and set off a nuclear crisis. REUTERS/Toru Hanai 

A woman attends a moment of silence at 2:46 p.m. (0546 GMT), the time when the magnitude 9.0 earthquake struck off Japan's coast in 2011, during a rally in Tokyo, Japan March 11, 2018, to mark the seventh-year anniversary of the earthquake and tsunami that killed thousands and set off a nuclear crisis. REUTERS/Toru Hanai 

A resident reacts as he attempts to extinguish a fire that broke out at the Kijiji slums in the Southlands estate of Nairobi, Kenya, January 28, 2018. REUTERS/Thomas Mukoya TPX IMAGES OF THE DAY 

A resident reacts as he attempts to extinguish a fire that broke out at the Kijiji slums in the Southlands estate of Nairobi, Kenya, January 28, 2018. REUTERS/Thomas Mukoya 

A banner reading "No borders" is seen on the snow as demonstrators march from Claviere in Italy to Montgenevre in France to ask for open borders for migrants, January 14, 2018. REUTERS/Massimo Pinca 

Migrants, part of a group intercepted aboard a dinghy off the coast in the Mediterranean Sea, are reflected in the sea as they arrive on a rescue boat at the port of Malaga, Spain January 13, 2018. REUTERS/Jon Nazca 

Snow covers the beginning of a mountain pass from where migrants try to cross from Italy into France, near the town of Bardonecchia in northern Italy, December 19, 2017. REUTERS/Siegfried Modola SEARCH "MODOLA ALPS" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

A sign that warns of avalanche dangers protrudes from the snow on a mountain pass where migrants try to cross from Italy into France, near the town of Bardonecchia in northern Italy, December 20, 2017. REUTERS/Siegfried Modola SEARCH "MODOLA ALPS" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

Migrants walk through snow after crossing part of the Alps mountain range from Italy into France, near the town of Nevache in southeastern France, December 21, 2017. REUTERS/Siegfried Modola SEARCH "MODOLA ALPS" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY. 

A man washes himself in a cold river after spending the night under a highway bypass from where migrants of mostly North African origins prepare to attempt to cross the Italian border into France, in the Mediterranean coastal town of Ventimiglia, northern Italy, December 4, 2017. REUTERS/Siegfried Modola SEARCH "MODOLA ALPS" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY. 

A woman dries her shoes under a high-speed railway bridge at a village, which is mainly inhabited by migrant workers, in Tongzhou district of Beijing, China December 16, 2017. REUTERS/Jason Lee 

A dog looks on amid the debris of demolished low-cost dwellings under a high-speed railway bridge at a village, which is mainly inhabited by migrant workers, in Tongzhou district of Beijing, China December 16, 2017. REUTERS/Jason Lee 

An abandoned potted plant is pictured amid the debris of demolished low-cost dwellings near apartment blocks under construction at a village, which is mainly inhabited by migrant workers, in Tongzhou district of Beijing, China December 16, 2017. REUTERS/Jason Lee 

Beds are seen in a room that used to be inhabited by migrant workers in a low-cost dormitory that has been cut off from water and electricity supply in Beijing, China, December 14, 2017. Picture taken December 14, 2017. REUTERS/Thomas Peter 

A candle stands on a table in a room that used to be inhabited by migrant workers in a low-cost dormitory that has been cut off from water and electricity supply in Beijing, China, December 14, 2017. Picture taken December 14, 2017. REUTERS/Thomas Peter 

Beds are seen in a room that used to be inhabited by migrant workers in a low-cost dormitory that has been cut off from water and electricity supply in Beijing, China, December 14, 2017. Picture taken December 14, 2017. REUTERS/Thomas Peter 

Belongings and floats of migrants, part of a group intercepted aboard a dinghy off the coast in the Mediterranean Sea, are seen inside a dinghy after arriving on a rescue boat at the Port of Malaga, Spain December 7, 2017. REUTERS/Jon Nazca 

Lunch leftovers are seen on the ground at a makeshift camp for refugees and migrants next to the Moria camp on the island of Lesbos, Greece, November 30, 2017. REUTERS/Alkis Konstantinidis SEARCH "MORIA CAMP" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

A blanket is left to dry on a rock at a makeshift camp for refugees and migrants next to the Moria camp on the island of Lesbos, Greece, December 2, 2017. REUTERS/Alkis Konstantinidis SEARCH "MORIA CAMP" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

The power lines cut by security guards of village are pictured at an apartment for migrant worker as residents are required to move out due to a citywide fire safety inspection prompted by a deadly fire in an apartment block at Xinjiancun, in Picun village on the outskirts of Beijing, China November 28, 2017. REUTERS/Jason Lee 

Sunflower seeds are pictured in a depot with Syrian fabrics and goods in Hamburg, Germany October 20, 2017. Picture taken October 20, 2017. REUTERS/Fabian Bimmer 

Gloves hang on a razor fence at the harbour at the ferry terminal in Ouistreham, France, October 30, 2017. REUTERS/Pascal Rossignol 

Sudanese migrants warm their hands over a wood fire in Ouistreham, France, October 30, 2017. REUTERS/Pascal Rossignol 

An Austrian army soldier and an Austrian police officer stand guard during border controls at a highway toll station in Schoenberg , Austria, August 23 , 2017. REUTERS/Dominic Ebenbichler 

A toy parrot is left behind in a former migrant home before its demolition at Dongsanqi village in the outskirts of Beijing, China, August 8, 2017. Picture taken August 8, 2017. REUTERS/Thomas Peter 

A migrant worker examines a lettuce at a farm in Kent, Britain July 24, 2017. Picture taken July 24, 2017. REUTERS/Neil Hall 

Migrants living on sidewalks near the reception center for migrants and refugees at porte de la Chapelle are seen during their eviction by French police, north of Paris, France, July 7, 2017. The word reads "equality". REUTERS/Pascal Rossignol 

The Vive La Casa shelter for migrants is seen in Buffalo, New York, U.S. July 5, 2017. Picture taken July 5, 2017. REUTERS/Chris Helgren 

A Myanmar migrant worker sorts shrimp at a wholesale market for shrimp and other seafood in Mahachai, in Samut Sakhon province, Thailand, July 4, 2017. REUTERS/Chaiwat Subprasom 

Myanmar migrant workers sort shrimp at a wholesale market for shrimp and other seafood in Mahachai, in Samut Sakhon province, Thailand, July 4, 2017. REUTERS/Chaiwat Subprasom 

Myanmar migrant workers sort shrimp at a wholesale market for shrimp and other seafood in Mahachai, in Samut Sakhon province, Thailand, July 4, 2017. REUTERS/Chaiwat Subprasom 

A stroller is left on the side of the road by the U.S.-Canada border on Roxham Road from Champlain, New York, U.S., April 24, 2017. REUTERS/Christinne Muschi SEARCH "MUSCHI CABBIE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

A group of migrants who said they were from Djibouti and Somalia follow railway tracks towards the Canada-U.S. border as seen from Emerson, Manitoba, Canada, March 27, 2017. Picture taken March 27, 2017. REUTERS/Chris Wattie 

A group of migrants who said they were from Djibouti and Somalia follow railway tracks towards the Canada-U.S. border as seen from Emerson, Manitoba, Canada, March 27, 2017. Picture taken March 27, 2017. REUTERS/Chris Wattie 

A group of migrants who said they were from Djibouti and Somalia walk along railway tracks after crossing the Canada-U.S. border in Emerson, Manitoba, Canada, March 27, 2017. Picture taken March 27, 2017. REUTERS/Chris Wattie 

A group of migrants who said they were from Djibouti and Somalia walk along a residential street after crossing the Canada-U.S. border in Emerson, Manitoba, Canada, March 27, 2017. Picture taken March 27, 2017. REUTERS/Chris Wattie 

Greek coast guard vessels take part in an operation after an inflatable boat carrying refugees and migrants sank and at least 15 people drowned, on the island of Lesbos, Greece April 24, 2017. REUTERS/Elias Marcou 

Migrants walk among railway wagons in the northern city of Thessaloniki, Greece, April 6, 2017. REUTERS/Alexandros Avramidis SEARCH "TRAINS MIGRANTS" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

A migrant eats at a bus station in front of the Tiburtina railway station in Rome, Italy, March 8, 2017. Picture taken March 8, 2017. REUTERS/Max Rossi 

A girl carries clothes that were hung to dry during snowfall at a refugee camp in Ritsona, northern of Athens, Greece, December 30, 2016. REUTERS/Alkis Konstantinidis 

Flowers and messages are seen attached on a tree near the site where a 19-year-old female student died on October 15, in Freiburg, Germany December 6, 2016. REUTERS/Vincent Kessler 

A woman passes by flowers and messages attached on a tree near the site where a 19-year-old female student died on October 15, in Freiburg, Germany December 6, 2016. REUTERS/Vincent Kessler 

Flowers and messages are seen attached on a tree near the site where a 19-year-old female student died on October 15, in Freiburg, Germany December 6, 2016. REUTERS/Vincent Kessler 

A migrant warms himself by the fire inside a derelict customs warehouse in Belgrade, Serbia, November 10, 2016. REUTERS/Marko Djurica SEARCH "MIGRANTS WAREHOUSE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

A migrant wrapped in a blanket sleeps on a floor inside a derelict customs warehouse in Belgrade, Serbia, November 10, 2016. REUTERS/Marko Djurica SEARCH "MIGRANTS WAREHOUSE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

Migrants wrapped in blankets sleep on a floor of a derelict customs warehouse in Belgrade, Serbia, November 10, 2016. REUTERS/Marko Djurica SEARCH "MIGRANTS WAREHOUSE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

An Eritrean migrant walks after arriving by plane from Italy at the first registration camp in Erding near Munich, Germany, November 15, 2016. REUTERS/Michael Dalder 

General view of a 13ft-high (4m) stone wall which is being built to separate a residents housing development from a migrant accommodation facility in Neuperlach outside Munich, November 8, 2016. REUTERS/Michael Dalder 

General view of a 13ft-high (4 m) stone wall which is being built to separate a residents housing development from a migrant accommodation facility in Neuperlach outside Munich, November 8, 2016. REUTERS/Michael Dalder 

A window of a migrant accommodation facility is seen though hole in a 13ft-high (4 m) stone wall which is being built to separate a residents housing development from potential refugees, in Neuperlach outside Munich, November 8, 2016. REUTERS/Michael Dalder TPX IMAGES OF THE DAY

A migrant walks past debris from makeshift shelters during the dismantlement of the camp called the "Jungle" in Calais, France, October 28, 2016. REUTERS/Pascal Rossignol