ISTANBUL (Reuters) - Heavy fog on Sunday forced the closure of Istanbul's Bosphorus Strait, a vital route for Russian oil and other commodities, to shipping traffic, agent GAC said.

Five tankers were waiting to travel north at the mouth of the narrow strait, which links the Black Sea with the Mediterranean, GAC said. Traffic at the Dardanelles at the other end of the Sea of Marmara was unaffected.

Reduced visibility due to poor weather in the winter frequently requires maritime officials to shut the Bosphorus, disrupting the flow of Russian oil and grain to world markets.

Each year about 150 million tonnes of crude and petroleum products pass through the waterway, which bisects Istanbul, a city of 15 million people.