Refiners on the U.S. West Coast and Hawaii have stepped up purchases of Russian crude, taking advantage of a narrow gap between U.S. and global prices as they look to guard against a seasonal shortage of Alaskan supply, trade and industry sources said. Up to four tankers were expected to carry nearly 3 million barrels of Russia's ESPO crude from Kozmino near the city of Vladivostok to refineries in the United States this month and next, the sources said.