Oil from a damaged tanker has reached South Korea's west coast, not far from a nature reserve, BBC reported, citing coast guards.
Emergency workers in Taean region are battling to contain the slick, now up to 20km (12 miles) long, from harming wildlife and valuable sea farms.
More than 10,000 metric 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.
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.
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.