At least seven people, two of them children, have drowned in the Aegean Sea while trying to cross from Turkey to Greece.
It was not immediately clear how many people were in the boat when it overturned near the island of Kos.
Separately - further north in the Aegean around Lesbos and Chios - rescuers plucked 164 people from the sea, Greece's national coastguard said.
Greek islands last year became the main gateway to Europe for people fleeing war zones in the Middle East, but also for so-called economic migrants fleeing poverty in Asia and Africa.
Around 850,000 arrived in 2015. The surge has not abated despite the danger of a winter crossing, even though many use rickety vessels.
The UN refugee agency UNHCR has registered more than 44,000 since the start of this year.
With the latest deaths, the toll from drowning among migrants rose above 200 in the eastern Mediterranean since the start of the year.
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