ANCHORAGE, Alaska — Salvage crews were working to transfer thousands of gallons of diesel fuel from a stricken tugboat that crashed against the same Alaskan reef that damaged the Exxon Valdez 20 years ago.

Efforts to remove the 125,000 litres of fuel were slowed Friday when a 1.5-kilometre-long diesel sheen spread across Prince William Sound.

Overflights found that the fuel had escaped a containment boom and was not the result of a new leak in the fuel tanks, so transfer work resumed later Friday, according to Jim Butler, a spokesman for the boat's owner, Crowley Maritime Services.

The work was expected to be completed Saturday.

Once the fuel from the Pathfinder is transferred to another vessel, the salvage team will have a good idea of how much diesel fuel spilled into the sound.

The 41-meter tug Pathfinder had just finished checking for dangerous ice and was heading back to port in Valdez when it hit Bligh Reef at 6:15 p.m. Wednesday, spilling diesel fuel from two damaged tanks.

The boat is part of the Ship Escort Response Vessel System that was created after the Exxon Valdez ran aground in 1989 and spilled nearly 41 million litres of crude oil -- the worst ever U.S. spill.

The accident drew a rebuke from U.S. Sen. Mark Begich of Alaska, who called it troubling that a spill response vessel "managed to run aground on one of the most well-marked and well-known reefs in the Northern Hemisphere."

The Coast Guard said that a dive team inspecting the tug found damage to the hull and a section of keel missing.

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.

Officials said that diesel fuel will evaporate in time.