The bodies of at least 24 people including children have been recovered on Monday after their boat sank in the Black Sea close to Istanbul.
Around 50 migrants, believed to be from Afghanistan and Syria, were on board along with a Turkish captain.
The boat is thought to have been heading to Romania but it was unclear where it had set off from.
Seven people are reported to have been rescued but scores are still missing.
A large rescue operation is now under way at the mouth of the Bosphorus, the river that runs through Istanbul, where it meets the Black Sea.
Since the beginning of the civil war in Syria, thousands of migrants have tried to reach the EU by boat from western and southern Turkey.