Greek rescuers recovered the bodies of six drowned migrants, including a young boy, Wednesday after their boat sank off Kos island, the coastguard said.
The bodies were found during a rescue operation that was launched after a survivor reported the sinking.
The man had managed to swim to an islet near Kos, where he raised the alarm.
Three Greek patrol boats, two vessels from EU border agency Frontex and an NGO boat took part in the operation, the coastguard said.
The tragedy comes just five days after 45 people including 20 children drowned off Kalolimnos near Kos on Friday.
More than a million people headed to Europe in search of new lives last year, most of them refugees fleeing conflict in Syria, Iraq and Afghanistan in the continent's worst migration crisis since World War II.
The onset of winter does not appear to have deterred the migrants, with boats still arriving on the Greek islands from Turkey daily.
The UN says more than 46,000 people have arrived in Greece so far this year, 31 times more than for all of January 2015.
The EU is mulling allowing states in the passport-free Schengen zone to reintroduce border checks for up to two years to cope with the migration crisis.
Greece's interior minister for migration Yiannis Mouzalas said such an extension would put Greece in a "difficult" situation by making it harder for new arrivals to continue on to northern Europe.