At least 20 illegal migrants have drowned when a boat taking them towards the European Union sank in the Black Sea just off Istanbul, Turkish media reported.

Six people were rescued and seventeen more were missing from the stricken boat, which had set off earlier from Istanbul and had travelled through the Bosphorus Strait on its way to Romania, the reports added on Monday.

Eleven children, five women and one man were still unaccounted for, as authorities continued their search near the northern entrance of the Bosphorus.

Al Jazeera's Bernard Smith, reporting from Istanbul, said the chances of finding survivors was slim in the rough and cold water.

Those on board were Afghans who had paid several thousand euros each in search of a better life in the EU and had set off earlier from Bakirkoy, an Istanbul suburb on the Sea of Marmara side of the Bosphorus.

"The boat could have capsized either because it was overloaded or because of the weather being too windy," our correspondent said.

"Fishermen who first went to the rescue said it was a small pleasure boat designed to carry perhaps seven to nine people on tours of the water around Istanbul. We know so far that more than 20 people have drowned and six have survived. Clearly the boat has been overloaded."

A coastguard official contacted by AFP news agency confirmed the accident but could not give the exact number of victims.

The boat capsized due to overloading and bad weather conditions.

Turkey has become a hub for illegal immigrants who seek to reach Europe in the search for a better life.

But the journey is frequently perilous, and hundreds of immigrants have drowned en route to Europe in recent years.

PHOTO CAPTION

Rescuers retrieve a boat that sank off the Black Sea village of Garipce near Istanbul November 3, 2014.

Al-Jazeera