Chinese authorities are working to contain an oil spill on the country's north-east coast as warnings emerge of a heavy long-term environmental impact.

The government has mobilised hundreds of fishing boats and other vessels to clean up the spill that occurred in the port city of Dalian.

But Greenpeace says many people thrown into the effort have been reduced to using their bare hands.

The spill happened last Friday after two pipelines exploded at an oil storage depot, triggering a spectacular blaze that burned throughout the weekend.

Shortly after the spill, officials said about 1,500 tonnes of oil were spilled into the Yellow Sea off Liaoning province.

A government estimate on Monday said the slick had affected 435 square kilometres of the Yellow Sea.

However a report late Wednesday said the slick had spread to 946 square kilometres, and stretched as far as 90 kilometres along the coast.

The report appeared in the Shenyang Evening News, based in the provincial capital.

The government says about 40 special oil-skimming vessels are leading the clean-up and that 23 tonnes of oil-eating bacteria are being employed.

Zhong Yu, a Greenpeace campaigner observing the clean-up efforts, says many of the mobilised civilians and firefighters had no equipment or protective gear.

"The citizens-turned-cleaners we saw yesterday in the sea basically did not have any protective gear and could only use their hands to clean up the oil," she said.

Ms Zhong said tourist beaches and other long stretches of coast are awash with the black sludge that is up to 30 centimetres thick in some places near the coast.

"There is a strong smell of acid and oil in the air," she said.

The economic impact is already being felt by businesses in the area, with state press reports saying tourist beaches normally crammed with summer visitors are empty.

Other beaches are closed to the public.

The area also has a major fishing industry, but catches have been banned for now.

A firefighter involved in clean-up efforts drowned on Tuesday after he entered the oil-slathered waters to try to clean a pump sucking up spilt oil, state media reports said.

Authorities have expressed confidence in corralling the spill and have repeatedly pointed out the reported volume of oil is minuscule compared to the US oil disaster in the Gulf of Mexico.