A boat carrying suspected migrants from Afghanistan and Syria sank Monday just north of the Bosphorus Strait off the coast of Istanbul, leaving at least 24 people dead. A further 12 were believed to be missing, while seven were rescued, Turkish authorities said.

Coast guard vessels, divers and passing fishing boats were scanning the waters for more survivors. Twelve children were believed to be among those on board, the coast guard said in a statement. It was unclear what caused the vessel to sink or where it had set sail from.

Col. Sakir Cicek, of the Turkish military's general staff command in the capital Ankara, said the people on board were believed to be from Afghanistan and Syria.

Emrecan Kolcu, one of the fishermen involved in the search and rescue operation, said he headed to the area when another fisherman radioed that he had seen a body in the sea.

"We saw dead bodies," Kolcu said. "It was not possible not to see them anyway; everywhere was full of dead bodies. They had life vests on them."

The Istanbul governorship office said the boat had issued a distress call early Monday morning about three miles (5 kilometers) north of the Bosphorus Strait in the Black Sea.