More than 10,000 tons of oil has leaked into the sea off South Korea, following a collision between an oil tanker and another vessel.
The collision ripped a hole in the tanker's side
Reports say the 146,000-ton tanker Hebei Spirit collided with a crane mounted on a barge, suffering a punched hole in her side.
Maritime officials say it is the country's worst oil spill.
More than 30 coast guard vessels and four helicopters are trying to prevent the slick from spreading.
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 ( 62 miles) south of the capital, Seoul. South Korea's previously largest spill happened in 1995, when 5,000 tonnes of oil washed onto the country's southern coast.