The South Korean government has declared a "state of disaster" along a stretch of coastline affected by the country's worst-ever oil spill.
Over 6,000 people are working to clear up the spill
A fleet of 100 ships has been fighting to contain the 10,000-ton spill, but emergency workers have been unable to prevent the oil washing ashore.
Earlier the Hebei Spirit, the tanker which was hit by a barge and holed, causing the spill, was finally sealed.
The 17km (10.6-mile) slick threatens wildlife and valuable sea farms.
'Dark brown' sea
According to South Korea's Ministry of Maritime Affairs, the Hong Kong-registered vessel had been at anchor when it was hit by the industrial barge, which had broken free from its towing lines.
MARITIME OIL SPILLS
July 1979 - Greek tanker Atlantic Express spilled 287,000 tons off Trinidad and Tobago
March 1978 - Amoco Cadiz spilled 223,000 tons off Brittany, France
November 2002 - Greek-owned Prestige spilled 77,000 tons off Galicia, Spain
March 1989 - Exxon Valdez spilled 37,000 tons in Prince William Sound, Alaska, US
December 2007 Hebei Spirit spills 10,000 tons off S Korea's west coast
Local residents reported seeing the oil slick as it approached the shoreline.
"This is not the sea we used to have. It's all dark brown. Sea waves are dark brown, the beach is scattered with greasy clumps of sand," a witness told the AFP news agency.
Coast guard, navy and fishing vessels, accompanied by six helicopters, have been trying to prevent the slick from spreading using a boom and chemical dispersants.
Over 6,000 people, including government workers, local residents and volunteers, are participating in the operation.
But they have been unable to prevent crude oil from washing onto the beaches in Taean district, 100km (60 miles) south-west of the capital Seoul.
They spent most of Sunday carrying buckets of pungent sludge, treating oiled birds and scrubbing blackened rocks with absorbent cloths.
The BBC's John Sudworth, in Seoul, says that with their beach now coated in a 10cm layer of oil, local people fear for their livelihoods and for the long-term impact on the region's ecology, including a designated national maritime park and areas of wetland important for migrating birds.
"We are worried about an ecological disaster," said Kim Jong-sik, an official with the ministry of maritime affairs and fisheries.
"We have set up a boom, trying to stop oil from spreading along the coast, but oil sometimes overflows it depending on the currents," he told the French news agency AFP.
"If we fail to contain the spread, it is feared [it will] inflict serious damage to the coast," he said.
South Korea's previously largest spill happened in 1995, when 5,000 tonnes of oil washed onto the country's southern coast.