In this photo released by Greenpeace, a firefighter rushes to aid his colleague who ran into trouble amid thick oil cover as they attempted to fix an underwater pump.* (AP) BEIJING -- China's worst known oil spill is dozens of times larger than the government has reported - bigger than the famous Exxon Valdez spill two decades ago - and some of the oil was dumped deliberately to avoid further disaster, an American expert said Friday. China's government has said 1,500 tons (461,790 gallons) of oil spilled after a pipeline exploded two weeks ago near the northeastern city of Dalian, sending 100-foot- (30-meter-) high flames raging for hours near one of the country's key strategic oil reserves. Such public estimates stopped within a few days of the spill. But Rick Steiner, a former University of Alaska marine conservation specialist, estimated 60,000 tons (18.47 million gallons) to 90,000 tons (27.70 million gallons) of oil actually spilled into the Yellow Sea. "It's enormous. That's at least as large as the official estimate of the Exxon Valdez disaster" in Alaska, he told The Associated Press. The size of the offshore area affected by the spill is likely more than 400 square miles (1,000 square kilometers), he added.