BEIJING, CHINA (AFP) - An oil spill in northeastern China may have been about 60 times bigger than the government reported, ranking it among the world's worst known oil disasters, an environmental group said Friday.

Greenpeace said between 60,000 and 90,000 tonnes of crude may have poured into the Yellow Sea after two pipelines exploded at an oil storage depot in the port of Dalian on July 16 - dwarfing official estimates of 1,500 tonnes.

"It's one of the 30 largest oil spills in human history," said Richard Steiner, a marine conservationist with the University of Alaska and an adviser to Greenpeace.

"If our estimates are correct, it is certainly larger than the Exxon Valdez spill in Alaska in 1989," Steiner told a news conference.

Official estimates said the spill covered more than 435 square kilometres (170 square miles) of water, but one Chinese media report last week said the slick had spread to 946 square kilometres.

Steiner, who spent one and a half days in Dalian assessing the spill, said the lack of adequate aerial surveillance meant no one really knew the extent of the slick, even suggesting it could have reached North Korea by now.

Greenpeace admitted its estimates could be off by 50 percent - either higher or lower - due to the lack of reliable data from the government and oil companies.

China said Monday the oil spill had been successfully controlled 10 days after the accident, as operations at Dalian port returned to normal.

The city's mayor Li Wancai said authorities had achieved a "decisive victory" in the battle to contain the spill, the state-run Dalian Daily reported.

Oil sludge in the sea has mostly been cleaned and the spill has not spread to international waters, the report said.

However, Steiner said there was still oil in the water and on beaches and clean-up efforts would need to continue throughout August and even into the autumn.

It could take the local environment 10 years, or even longer, to recover from the disaster, he added.