An oil slick almost five kilometres long spread over the water off the south coast of Alaska on Thursday after a tugboat carrying 125,000 litres of diesel fuel ran aground on a reef.

The 42-metre tug, which was put in service to help prevent another oil spill disaster in Prince William Sound, ran aground on the same reef as the Exxon Valdez 20 years ago in what remains the worst oil spill in U.S. history.

The U.S. Coast Guard said the tug with six crew aboard had just completed an ice survey and was heading back to port in Valdez when it grounded on Bligh Reef. It reported the grounding in a radio call at 6:15 p.m. PT Wednesday.

Two of the tug's fuel tanks were damaged. The tanks contained an estimated 125,000 litres of diesel fuel, about a quarter of their capacity.

Coast Guard Chief Petty Officer Dana Warr said some fuel remained in the two tanks, but the amount wasn't known. A barge was being brought to the scene to prepare off-loading the fuel.

A fuel sheen nearly five kilometres long and about 30 metres wide had drifted away from the vessel. There was no sheen visible around the tug.

Coast Guard Petty Officer Jon-Paul Rios said the tug's crew deployed about 65 metres of fuel containment booms around the vessel after getting clear of the reef and continuing to deeper waters.

Crew, coast guard acted quickly

The crew and coast guard acted quickly, Rios said.

"We jumped on this pretty fast due to the Exxon-Valdez and the training we got from that," he said.

A dive team conducting a hull inspection found extensive damage to the Crowley Marine Services tug. A one- to two-metre section of the keel also was missing.

The tug, called the Pathfinder, is part of the Ship Escort Response Vessel System, which was put in place following the 1989 disaster when the 300-metre Exxon Valdez ran aground and spilled about 41 million litres of crude oil into Prince William Sound.

The SERVS system provides two escort tugboats for each tanker travelling through the sound after leaving the Valdez Marine Terminal with North Slope crude delivered through the trans-Alaska pipeline.

Rios said while any spill is concerning, the grounding of the tug is different from the Exxon Valdez accident, in which the tanker spilled an enormous amount of black crude oil.

The tug was carrying diesel fuel, a much lighter fuel that will evaporate in time, Rios said. The coast guard estimates that for a spill of up to 20,000 litres of diesel fuel, it takes a day or less to evaporate.

This spill will take somewhat longer, but it will evaporate, he said.

The tug's six crew members were tested for alcohol use. The tests were negative.

With files from The Associated Press