Beijing - The sinking of an Iranian tanker off China could severely damage marine life, environmental experts warned Monday, as state media reported an oil spill covering up to 50 square miles at the scene.

The Sanchi, carrying 136,000 tonnes of light crude oil from Iran, went under on Sunday after a new and massive fire erupted, sending a cloud of black smoke as high as one kilometre above the East China Sea.

The bodies of only three of the 32 crew members have been found since the vessel collided with the CF Crystal, a Hong Kong-registered bulk freighter, on January 6, sparking a fire that Chinese rescue ships struggled to extinguish.

Iranian officials said there was no hope of finding survivors among the crew of 30 Iranians and two Bangladeshis, prompting grief and anger among families of the sailors in Tehran.

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Tanker ablaze, 32 missing after collision off China

"Oil spills from the ship are still burning" at the scene, China's transport ministry said Monday.

An oil spill 11.5 miles long and up to 4.6 miles wide surrounded the site as of Monday afternoon, according to the official People's Daily newspaper.

Dramatic photos showed a huge column of black smoke rising from bright red flames.

Alaska-based oil spill consultant Richard Steiner called the accident "the single largest environmental release of petroleum condensate in history".

"Given the poor condition of the hull of the ship after a week of explosions and fire, it is my assumption that none of the cargo holds or fuel compartments remain intact, and thus all of the condensate and fuel has been released," Steiner told AFP.

Even if only 20 per cent of the vessel's cargo was released into the sea, it would still be an amount about equivalent to Alaska's disastrous 1989 Exxon Valdez crude oil spill, he said.

"I don't know of any condensate spill into a marine environment larger than 1,000 tonnes, and most that we know of have been less than one tonne," he said.

The Sanchi's own fuel tank was able to accommodate some 1,000 tonnes of heavy diesel, according to Chinese media.

'Worst situation'

On Sunday state broadcaster CCTV cited Zhang Yong, a senior engineer with China's State Oceanic Administration, as playing down environmental concerns.

The Sanchi's light crude would have "less impact on the ocean" than other kinds of oil, and "minimal" impact on humans, given how far the incident occurred offshore, he said.

But the sinking of the ship before more oil had a chance to burn off was "the worst situation" possible, Ma Jun, director of the Institute of Public and Environmental Affairs, told the Global Times newspaper.

"The condensate oil, a kind of ultra-light oil on Sanchi, is different than other types of crude oil and is poisonous to marine life," he warned.

Unlike crude, condensate does not form a traditional surface slick when spilt. Instead, it generates a toxic underwater plume of hydrocarbons invisible from the sea surface.

Whales, porpoises, seabirds, fish, and plankton in contact with these hydrocarbons in the East China Sea will either die quickly or develop "sub-lethal injuries" such as physiological impairment, reproductive failure and chronic diseases, said Steiner.

The region is also a crucial spawning site for many large fish species, whose eggs and larvae have "undoubtedly been exposed" to the toxic compounds, he said.

"Just because there is no traditional surface slick does not mean there is minimal impact. While the toxic phase of the spill may only last a few months, the injury to populations could persist much longer," he said.

He slammed governments for failing to gather environmental data more quickly.

"As no one has been conducting a scientific assessment of (the environmental impact), the governments and ship owners are likely to claim, erroneously, there was limited damage."