Oil from a damaged tanker has reached South Korea's west coast, not far from a nature reserve, coast guards report.
Local residents rushed to help the clean-up
Emergency workers in Taean region are battling to stop the slick, now up to 20km (12 miles) long, from harming wildlife and valuable sea farms.
More than 10,000 tons of oil began leaking into the sea after the 146,000-ton Hebei Spirit oil tanker collided with a barge.
Maritime officials say it is the country's worst oil spill.
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.
Dozens of coast guard vessels and six helicopters are trying to prevent the slick from spreading using a boom and chemical dispersants, but it is currently drifting in strong winds.
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.
An emergency operation was quickly launched.
"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.
The oil threatens a stretch of scenic coastline 100 km (60 miles) south-west of the capital, Seoul.
The area includes a designated national maritime park and provides important wetland stopovers for migrating birds.
South Korea's previously largest spill happened in 1995, when 5,000 tonnes of oil washed onto the country's southern coast.