Chinese vessels clean up crude oil in the sea near Dalian, Liaoning province, on Monday. (China Daily/Reuters)

China's largest reported oil spill emptied beaches along the Yellow Sea as its size doubled Wednesday, while cleanup efforts included straw mats and frazzled workers with little more than rubber gloves.

An official warned the spill posed a "severe threat" to sea life and water quality as China's latest environmental crisis spread off the shores of Dalian, once named China's most livable city.

One cleanup worker has drowned, his body coated in crude.

"I've been to a few bays today and discovered they were almost entirely covered with dark oil," said Zhong Yu with environmental group Greenpeace China, who spent the day on a boat inspecting the spill.

"The oil is half solid and half liquid and is as sticky as asphalt," she told The Associated Press by telephone.

The oil had spread over 430 square kilometres of water five days since a pipeline at the busy northeastern port exploded, affecting oil shipments from part of China's strategic oil reserves to the rest of the country. Shipments remained reduced Wednesday.

State media have said no more oil is leaking into the sea, but the total amount of oil spilled is not yet clear.

Greenpeace China released photos Wednesday of inky beaches and of straw mats about two square metres in size scattered on the sea, meant to absorb the oil.

Fishing in the waters around Dalian has been banned through the end of August, the state-run Xinhua News Agency reported.

"The oil spill will pose a severe threat to marine animals, and water quality, and the sea birds," Huang Yong, deputy bureau chief for the city's Maritime Safety Administration, told Dragon TV.

Firefighter drowns during cleanup

At least one person died during cleanup efforts. A 25-year-old firefighter, Zhang Liang, drowned Tuesday when a wave threw him from a vessel, Xinhua reported.

Officials, oil company workers and volunteers were turning out by the hundreds to clean blackened beaches.

"We don't have proper oil cleanup materials, so our workers are wearing rubber gloves and using chopsticks," an official with the Jinshitan Golden Beach Administration Committee told the Beijing Youth Daily newspaper, in apparent exasperation.

"This kind of inefficiency means the oil will keep coming to shore.… This stretch of oil is really difficult to clean up in the short term."

But 40 oil-skimming boats and about 800 fishing boats were also deployed to clean up the spill, and Xinhua said more than 15 kilometres of oil barriers had been set up to keep the slick from spreading.

China Central Television earlier reported an estimate of 1,360 tonnes of oil has spilled. That would amount roughly to 1.5 million litres — as compared with 356 million to 697 million litres in the BP oil spill off the U.S. coast.

China's State Oceanic Administration released the latest size of the contaminated area in a statement Tuesday.

Cause of explosion unknown

The cause of the explosion that started the spill was still not clear. The pipeline is owned by China National Petroleum Corp., Asia's biggest oil and gas producer by volume.

Friday's images of 30-metre-high flames at China's second largest port for crude oil imports drew the immediate attention of President Hu Jintao and other top leaders.

Now the challenge is cleaning up the greasy plume.

"Our priority is to collect the spilled oil within five days to reduce the possibility of contaminating international waters," Dalian's vice-mayor, Dai Yulin, told Xinhua on Tuesday.

But an official with the State Oceanic Administration has warned the spill will be difficult to clean up even in twice that amount of time.

Some locals said the area's economy was already hurting.

"Let's wait and see how well they deal with the oil until Sept. 1, if the oil can't be cleaned up by then, the seafood products will all be ruined," an unnamed fisherman told Dragon TV. "No one will buy them in the market because of the smell of the oil."