Istanbul - Twenty-seven people including 11 children drowned on Monday after their boat sank in the Aegean Sea while trying to cross from Turkey to Greece, the Turkish coastguard said.
The coastguard said it discovered the boat, carrying 40 people, half capsized after it set off from Edremit in the western province of Balikesir in an apparent bid to reach the Greek island of Lesbos.
So far, four migrants have been rescued both by air and by sea in a search and rescue operation, the coastguard said in a statement on its website.
Nine people were still missing.
The nationalities of the victims were not immediately known.
Dogan news agency initially said at least 35 migrants had died in two separate accidents off western Turkey, but later reported that there was a single incident, with 11 children dead.
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Turkey, which is hosting at least 2.5 million refugees from Syria's civil war, has become the main launchpad for migrants fleeing conflict, persecution and poverty to Europe.
The deaths came as German Chancellor Angela Merkel held talks in Ankara on reducing the influx of migrants to Europe, as about 30,000 Syrians remained stranded at the Turkish border after fleeing a regime offensive on the northern Syrian region of Aleppo.
Merkel said Turkey and Germany would seek the use of NATO resources to help police the Turkish coast to try to stop the people smugglers.
Turkish Prime Minister Ahmet Davutoglu, whose country has taken in 2.7 million people fleeing the Syrian conflict, said Ankara could not handle the burden alone.
The Turkish government struck a deal with the EU in November to halt the outflow of refugees, in return for three billion euros ($3.2 billion) in financial assistance.
But the deal and wintry weather in the Mediterranean do not appear to have deterred the migrants, with boats still arriving on the Greek islands daily.
The International Organization for Migration (IOM) said the number of refugees and migrants who perished in the Mediterranean in January alone topped 360.
In January, almost 62,200 migrants and refugees entered Europe through Greece, according to the IOM, most of them from Syria, Afghanistan and Iraq.