The two straits linking the Black Sea and the Mediterranean were "totally shut to sea traffic" as of Thursday at 16:01 pm (1400 GMT) but expected to resume sometime Friday evening, said Recep Canpolat, a spokesman for Turkey's maritime security office (KEGKI).

"On the spot, visibility was so bad you could not see a light signal 15 meters (50 feet) away," he said.

Turkish weather forecasts said the fog was expected to dissipate later Friday followed by rain on Saturday.

Heavy fog has forced officials to close the two sea lanes on and off since Tuesday, causing blockups at both ends.

Some 130 vessels were stuck Friday at the northern, Black Sea entrance to the Bosphorous, while another 70 were waiting the go-ahead at its southern end on the Marmara Sea, the KEGKI spokesman said.

Another 20 vessels were waiting for the Dardanelles, which links the Marmara and the Aegean Sea, to be reopened, he added.

The two sea lanes are the main passageway for Ukrainian and Russian oil tankers and cargo ships traveling from the Black Sea to other points in the world.