SEOUL - Oil that spilled from a punctured supertanker off the South Korean coast has drifted in the form of tar to a distant island, as tens of thousands of people continued massive cleanup operations Saturday.

Lumps of tar were spotted in waters near Yeondo, an island some 120 kilometres south of the site of South Korea's largest-ever oil spill that released about 66,000 barrels into the ocean Dec. 7, the coast guard said in a statement.

Officials said such oil lumps pose much less threat to the environment and are easier to remove than oil in liquid form.

"It's something like asphalt, and the lumps that were found near Yeondo are very small in numbers,'' said Lee Won-yol, an official at the main coast guard office overseeing clean-up operations. "We believe they pose little contamination threat and can be easily removed.''

The spill, which involved more than twice as much oil as South Korea's worst previous spill in 1995, has spoiled 339 seafood farms and contaminated 167 kilometres, including scenic beaches, said National Emergency Management Agency.

The accident happened when a crane-carrying barge lost control and slammed into the supertanker Hebei Spirit.

On Saturday, more than 40,000 troops, public officials and residents worked at polluted beaches. The coast guard also mobilized 313 vessels, including private fishing boats, and 17 helicopters.