HANOI, Feb 3 (Reuters) - Hundreds of Vietnamese including soldiers are working against the clock to clean up oil pollution soiling a popular tourist beach, but the source of the spill remains a mystery, an official said on Saturday.

"We are using all forces including the army, youth, students and militia to work on the beach," said Nguyen Su, a senior official in the ancient town of Hoi An, a UNESCO heritage site.

The spill blackened part of Danang beach and all 125 km (78 miles) of coastline of Quang Nam province including Hoi An, spurring residents to start picking up the oil-soaked sand by hand.

"We have to do it quickly, the oil can melt in the sun while waves are bringing more oil to the beach," said Su, who heads Hoi An's municipal Communist Party office.

"Tourists cannot go swimming now, the water's dirty and sticky," he said by telephone.

The National Committee for Search and Rescue, which is investigating the spill, has ruled out a barge that stuck in waters off Hoi An as the source, the Lao Dong (Labour) newspaper said on Saturday. More oil had been spotted drifting towards shore, the paper said.

The committee called on the beach cleaners, mainly residents, to hand over the soiled sand for proper disposal rather than bury it under the beach and further pollute the environment.

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