Rescuers pulled 24 bodies from the sea at the mouth of Istanbul's Bosphorus strait on Monday and rescued seven people after the sinking of a boat carrying migrants including children, the Turkish Coastguard Command said.
Tens of thousands of migrants from Africa, the Middle East and beyond pack into often unsafe boats each year and thousands of them drown in their efforts to enter the European Union through coastal states.
The boat, which capsized, was loaded with 42 Afghan illegal migrants, including 12 children and seven women, along with a Turkish captain, the Hurriyet news website reported. It was believed to have been heading for Bulgaria or Romania, but it was unclear where it set to sea.
Bodies covered in blankets were laid on a jetty on the European side of Istanbul's Bosphorus strait, a Reuters witness said.
"They had life jackets. But there were bodies everywhere. Babies, children... We pulled out 15-20 bodies," Hurriyet quoted fisherman Kadir Sert as saying.
Seven coastguard vessels and a helicopter were continuing search operations in the Black Sea, some 3 miles (5 km) north of the Bosphorus, the coastguard said in a statement.
Shipping agents GAC said the boat had been heading for the Romanian port of Constanta when it sank around 5 a.m. (0300 GMT) and was believed to be carrying around 50 refugees.
The governor's office said a diving team had also been sent to the area. There was no official comment on the number of people travelling on the vessel.
Most of the migration to Europe happens via the Mediterranean Sea and the International Organization for Migration (IOM) said last week that an estimated 3,200 migrants had died attempting to cross the Mediterranean this year.
"Many of them (are) victims of ruthless criminal gangs seeking to profit from the misery of men, women and children fleeing conflict and oppression," the IOM said in a statement on its wesbite.
Some 150,000 "irregular" migrants, many of them from the most troubled nations in Africa and the Middle East, had arrived safely in Europe over the past 10 months, it said.
The numbers of migrants have increased since "Arab Spring" uprisings triggered unrest across North Africa and civil war in Syria.
Traffic on the Bosphorus strait, one of the world's busiest waterways, was suspended at 10:20 am (0820 GMT) due to the search and rescue operations and resumed at 1100 GMT, GAC said.
The Bosphorus is a vital route for Russian oil and other commodities as the only outlet to the world's oceans from the Black Sea. It bisects Istanbul, a city of around 15 million people. Poor weather frequently forces its closure in winter.