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 461,790 gallons of oil spilled after a pipeline exploded two weeks ago near the northeastern city of Dalian, sending 100-foot-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 18.47 million to 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, he added.

The estimates, though rough, could complicate China’s efforts to move on from its latest environmental disaster: Dalian’s mayor already declared a “decisive victory” in the oil spill cleanup, state media reported.

The spill has caused at least one death when a cleanup worker drowned in the sticky crude, and thousands of Dalian residents have used everything from their bare hands to chopsticks to pick the goo from the sea.

Steiner, who worked on the 1989 Exxon Valdez spill, announced the China estimates after touring the oil spill area as a consultant for the environmental group Greenpeace China.

“It’s habitual for governments to understate oil spills,” Steiner said. “But the severity of the discrepancy is unusual here.”

An official with Dalian’s propaganda department told The Associated Press he was not aware of Steiner’s estimates and had no comment.

“I think we should follow the figures released by the city government,” said the man, who gave his surname as Li.

The government has said the pipeline exploded July 16 after workers continued to inject an agent to strip sulfur from oil after a tanker had finished unloading its cargo.

According to Steiner, firefighters at the scene later told Greenpeace China that workers had let oil escape from other tanks in the area to reduce the risk that another nearby tank containing the chemical dimethylbenzene would explode as well.

The oil terminal is owned by China National Petroleum Corp., Asia’s biggest oil and gas producer by volume.

Steiner said his estimates came from the fact the oil storage tank that was destroyed had a capacity of about 27.70 million gallons and reportedly had just been filled by the tanker.

He said his lower estimate of 18.47 million gallons came from the rate of oil recovery by thousands of fishing boats dispatched for the cleanup.

“They’ve already collected more oil than the official estimate of the spill size,” he told The Associated Press.

He praised the makeshift cleanup efforts: “Very low-tech. The thing is that it worked.”