A tugboat struck the same reef as the Exxon Valdez tanker 20 years ago, spilling diesel into Alaska's Prince William Sound and creating a three-mile-long slick, the US Coast Guard said on Friday.

An unknown quantity of the fuel leaked from the Pathfinder tug after it ran aground Wednesday on Bligh Reef. The boat's owners were pumping the remaining diesel from the original 33,500 gallons (127,000 liters) in its tanks.

Flyovers by a C-130 cargo plane and helicopters revealed "a light grey or silver diesel sheen spanning an area approximately three miles (five kilometers) long and 30 yards (meters) wide approximately one mile east of Glacier Island," the Coast Guard said on its website.

The tug had been scouting shipping lanes for ice when it struck the same rock that did for the Exxon Valdez on March 24, 1989, spilling 11 million gallons of crude into the sea in the worst US oil disaster.

Wednesday's accident was nothing like on the same scale and the Coast Guard said the slick was "rapidly dissipating" and not expected to hit the shoreline of Glacier Island for some time.

Experts surrounded the boat with containment booms to prevent the slick from spreading, while a recovery vessel, the Valdez Star, was using oil skimmers to try and recover spilled fuel.

"There's no recoverable sheen," Jim Butler, a spokesman for Crowley Maritime Service which owns the tug, told the Anchorage Daily News (ADN).

When the remaining fuel is removed, officials will be able to estimate how much spilled into the sound, the Coast Guard said.

Butler said it was not clear how the tug ran aground, ADN reported, but a navigational error at such a well-known spot left some people dumbstruck.

"Like most Alaskans, we at the Prince William Sound Regional Citizens Advisory Council are baffled as to how the Pathfinder managed to hit perhaps the most famous navigational hazard in the world -- Bligh Reef -- in conditions of relatively mild weather," council president Steve Lewis wrote in a blog posting.

The accident raises questions "about how well the painful lessons of the Exxon Valdez spill of 1989 have been learned by today's mariners," he said.

The Coast Guard said tests administered to the Pathfinder crew late Wednesday found no alcohol, ADN reported.

The captain of the Exxon Valdez, Joe Hazelwood, had been drinking on board the tanker before the 1989 wreck.