Thirteen refugees, including six children, have died after their inflatable dinghy collided with a cargo ship.
A Turkish coastguard source told Reuters a boat carrying 46 refugees towards the Greek island of Lesbos capsized on Sunday after the crash.
The coastguard managed to rescue 20 people, seven of whom are now receiving treatment, but another 13 are missing.
A Greek coastguard spokeswoman said: "They (the migrants) told rescuers there were 46 people in the inflatable dinghy in total.”
Rescues and sinkings occur almost daily in the often choppy seas off Greece's eastern islands.
Tens of thousands of mainly Syrian refugees have braved the short crossing from Turkey this year, mainly in flimsy and overcrowded inflatable boats.
A girl believed to be aged five died on Saturday and 13 others were feared drowned after their boat sank off the same island, a favoured entry point on a route that takes the migrants through the Balkans towards northern Europe.
Fifteen children were among 34 who died a week ago when their boat capsized off the small island of Farmakonisi.