TOKYO (AFP) - "Oil-like" blobs are washing up on the beaches of several southern Japanese islands, officials said on Friday (Feb 2), raising fears they could be from a tanker that sank in the area nearly three weeks ago.

The Iranian tanker Sanchi - carrying 111,000 tonnes of light crude oil - went under in a ball of flames on Jan 14 in Japan's economic waters in the East China Sea, sparking concerns it could lead to a massive environmental catastrophe.

Japan's government is analysing the origin of the "oil-like" substance, and has dispatched the coast guard to help with the clean-up, top government spokesman Yoshihide Suga said.

Local officials have reported the substance washing up along a 7km stretch of the island of Takarajima in recent days.

"It is still uncertain whether this is related to the Sanchi. We are currently collecting and analysing samples," Suga told a regular briefing in Tokyo.

"We are taking measures to contain (contamination) and dispatching patrol vessels and planes while working closely together with China and other nations."

Officials on other southern Japanese islands also reported the substance washing up on their shores.

"You can see that it is greasy sludge," said Wataru Higo, an official in Toshima, a village on Takarajima island.

"Our fear is that it might increase over time, depending on directions of the tide and wind," he told AFP.

"Flying fish lay eggs in our port in April and May. I wonder what kind of impact this will have on that."

The Sanchi caught fire after colliding with a bulk freighter in early January, setting off a desperate rescue mission by authorities. The bodies of only three of its 32 crew have been found so far.

Environmentalists and some scientists had warned that the spill could affect Japanese beaches.

Greenpeace scientist Paul Johnston urged authorities to boost clean up efforts and monitoring of regional waters.

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"Cetaceans and birds are at high risk of exposure, and fish may be contaminated as well," he said in a press statement on Friday.

"In order to confirm that (the substance) is from the Sanchi, it would need to be analytically 'fingerprinted' against a sample of the fuel oil taken from the site where the tanker went down."

Japanese and Chinese authorities played down the risk of environmental damage in the wake of the spill, saying efforts were being made to break up the fuel at sea and that it was also evaporating.

The type of condensate oil carried by the Sanchi does not form a traditional surface slick when spilt, but is nonetheless highly toxic to marine life and much harder to separate from water.