Winter could bring death to asylum seekers stranded on crowded Greek islands with only summer tents for shelter, aid groups said on Wednesday, urging a mass relocation to the mainland.

More than 10,000 people, mostly Syrians and Iraqis fleeing years of war, have massed on the Greek islands that lie closest to Turkey, since the European Union agreed a deal with Ankara in March 2016 to shut down the route through Greece.

Authorities say the terms of the agreement prevent them from traveling to the Greek mainland until their asylum applications are processed. Those who do not qualify are deported.