The Istanbul channel was closed to both directions of traffic at 8:05 a.m. (0605 GMT), GAC said in an e-mail statement. It was not clear when the strait will re-open. Four tankers were scheduled to transit the Bosphorus on Saturday, the agent said. Weather conditions were clear at the Dardenelles Strait at the other end of the Sea of Marmara and the channel remained open, it said. The Bosphorus and Dardenelles are the only maritime outlet for Black Sea countries shipping oil, grains and other commodities to world markets. Traffic disruptions are common in winter in the straits as poor weather makes it difficult for tanker captains to traverse the narrow, environmentally sensitive waterways. The delays can raise Black Sea crude grades and world tanker rates.