BEIJING — China's largest reported oil spill more than doubled in size to 430 sq. kilometers by Wednesday, forcing nearby beaches to close and prompting one official to warn of a "severe threat" to sea life and water quality.

The oil slick started spreading five days ago when a pipeline at a busy northeastern port exploded, sparking a massive fire that took more than 15 hours to contain. Hundreds of boats have been deployed to help with the cleanup.

At least one person has been killed in those efforts, a 25-year-old firefighter, Zhang Liang, who drowned Tuesday after a wave threw him from a vessel and pushed him out to sea, the state-run Xinhua News Agency reported. Another man who also fell in was rescued.

Beaches near Dalian, once named China's most livable city, were closing as oil started reaching their shores, Xinhua 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 Dalian, China Maritime Safety Administration, told Dragon TV.

The environmental group Greenpeace China released several photographs this week showing oil-slicked rocky beaches, a man covered in thick black sludge up to his cheekbones. One worker, covered in oil, was being carried away by a colleague, but he was not identified.

The amount of oil spilled in the explosion was still not clear Wednesday, though China Central Television earlier reported an estimate of 1,500 tons. That would amount roughly to 1,500,000 litres.

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

Though the slick has continued to expand -- it covered a 180-sq.-kilometer stretch earlier this week -- officials maintain no more oil was leaking into the Yellow Sea.

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

Images of 30-meter-high flames shooting up near part of China's strategic oil reserves late Friday drew the immediate attention of President Hu Jintao and other top leaders. Now the challenge is cleaning up the greasy brown 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.

The Dalian port is China's second largest for crude oil imports, and last week's spill appears to be the country's largest in recent memory.