Eight migrants drowned and at least another three were missing after their inflatable boat sank off the Greek island of Kos early on Tuesday, the coastguard said.
Seven people were rescued, and emergency crews were still trying to recover all the bodies of the people who died from the capsized vessel, a coastguard official added.
More than 600,000 refugees and migrants fleeing Syria, Afghanistan and other war-torn countries have arrived in Greece from Turkey this year, risking the short but perilous sea crossing, often on inflatable rafts. Hundreds have drowned.
Greece, struggling with its own economic crisis, has said it does not have the funds to care for all the arrivals before they move on to other destinations including Germany and Sweden.
Prime Minister Alexis Tsipras has said financial aid could be offered to Turkey to register and process the migrants on its territory instead. Tsipras is visiting Turkey on Tuesday and Wednesday to discuss the worst refugee crisis since World War Two.