SEOUL, Dec 8 (Reuters) - South Korean workers using oil skimmers and containment fences battled on Saturday to clean up the worst oil spill in the country's history and stop the slick from reaching a nature preserve on its west coast.

A Hong Kong-registered tanker began leaking an estimated 10,500 tonnes of crude oil on Friday after a barge carrying a crane slammed into it while the tanker was anchored off Daesan port about 110 km (70 miles) southwest of Seoul.

The slick was about 13 kms (7 miles) long about a day after the spill and was approaching Mallipo Bay, about 90 km southwest of Seoul, a maritime ministry official said.

"We're installing oil-containment fences to prevent further inflow," said Song Myeong-dal, head of the Information and Policy Monitoring team at the Ministry of Maritime Affairs and Fisheries.

The slick was about a day or two away from hitting an area on the west coast that has marine farms and oyster beds, Song said. "We are taking all measures to prevent that from happening."

Heavy winds and high waves hurt oil containment efforts on Friday but seas were calmer on Saturday.

The leak is about a third of the size of the 1989 Exxon Valdez spill of crude oil onto Alaskan shores, which was the costliest on record.

That clean-up alone from that disaster cost around $2.5 billion while the total costs, including fines and settlement of claims, were an estimated $9.5 billion.

South Korea has dispatched 23 naval vessels, 34 oil skimmers, six helicopters and 67 patrol ships to help contain the slick, Song said.

The Taean region, near where the spill took place, is popular for its beaches and is home to a national maritime park. It is also an important refuelling stop for migratory birds.

The very large crude carrier (VLCC) Hebei Spirit was about 5 miles outside the port, waiting to unload its cargo of some 260,000 tonnes of crude oil from the Middle East when it was struck by the barge.

Technical managers of the MT Hebei Spirit said in a statement on Friday the crane punched holes in three of the tanker's tanks. (Writing by Jon Herskovitz; Editing by Bill Tarrant)

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