A crane-carrying vessel collided with an oil tanker off South Korea's west coast on Friday, causing more than 66,000 barrels of crude oil to spill in what was believed to be South Korea's largest offshore oil leak, officials said.

Officials at the Maritime and Fisheries Ministry, citing coast guard reports, initially said about 110,000 barrels had leaked from the Hong Kong-registered tanker. But the coast guard later said additional information indicated the amount was significantly lower — about 66,043 barrels.

The accident occurred around 7:30 a.m. local time when the crane-carrying vessel slammed into the tanker Hebei Spirit in waters eight kilometres off Mallipo beach, 130 kilometres southwest of Seoul.

The extent of the spill's potential damage to communities or wildlife in the area was not immediately clear, a ministry official said on customary condition of anonymity citing office policy.

Jung Se-hi, a coast guard officer, said it dispatched 34 coast guard, navy and other government vessels as well as four helicopters to stop the spread of the spill and collect the oil.

He said an emergency floating fence was also set up around the scene but added it would take at least several days to complete the operation.

The coast guard said in a statement that the collision left holes in three containers aboard the tanker. Spills from two containers were later stopped, but oil was still leaking from the third container.