China rushed to keep an oil spill from reaching international waters, while an environmental group tried to assess if the country's largest reported spill was worse than has been disclosed.

Crude oil started pouring into the Yellow Sea off a busy northeastern port after a pipeline exploded late last week, sparking a massive 15-hour fire. The government says the slick has spread across a 70-square-mile (180-square-kilometer) stretch of ocean.

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

Images of 100-foot-high (30-meter-high) flames shooting up near part of China's strategic oil reserves drew the immediate attention of President Hu Jintao and other top leaders. Now the challenge is cleaning up the greasy brown plume floating off the shores of Dalian, once named China's most livable city.

The environmental group, Greenpeace China, shot several photographs at the scene Tuesday before their team was forced to leave. They showed oil-slicked rocky beaches, a man covered in thick black sludge up to his cheekbones, and workers carrying a colleague covered in oil away from the scene.

The state-run Xinhua News Agency reported a 25-year-old firefighter, Zhang Liang, drowned Tuesday after a large wave pushed him into the sea amid the clean up. Another man who also fell in was rescued. It was not immediately clear if either were the ones shown in the Greenpeace photos.

Activists said it was too early to tell what impact the pollution might have on marine life.

Officials told Xinhua they did not yet know how much oil had leaked, but China Central Television reported no more pollution, including oil and firefighting chemicals, had entered the sea Tuesday. It was not clear how far the spill was from China's closest neighbor in the region, North Korea.

Oil-control boats

Dalian's vice mayor, Dai Yulin, told Xinhua 40 specialized oil-control boats would be on the scene along with hundreds of fishing boats. Oil-eating bacteria were also being used in the cleanup.

"Our priority is to collect the spilled oil within five days to reduce the possibility of contaminating international waters," he said.

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.

"In terms of what is known to the public, this is definitely the biggest," said Yang Ailun, spokeswoman for Greenpeace China.

"Government and business leaders have been telling the media that there's no environmental impact. From Greenpeace's perspective, that's very irresponsible," she added. "It's too early to tell. Oil is still floating around."

While the Chinese public has not seized on the accident as its own version of the massive BP spill in the United States, warnings over the country's increasing dependence on oil were clear.

The International Energy Agency said Tuesday that China has overtaken the United States as the world's largest energy consumer, using the equivalent of 2.252 billion tons of oil last year. China immediately questioned the calculation.