ANCHORAGE, Alaska - A mile-long diesel sheen spread across Alaska's Prince William Sound yesterday where a tugboat ran aground near the site of the 1989 Exxon Valdez disaster, the Coast Guard said.

Officials had hoped to remove the fuel from the tugboat's tanks early yesterday before towing it back to Valdez, but diesel removal was halted after about 10 minutes when workers noticed a new sheen on the water's surface, Coast Guard Lt. Erin Christensen said.

He said helicopter flights measured the sheen at 50 feet wide by one mile long.

The 136-foot 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.

The boat is part of the Ship Escort Response Vessel System (SERVS) that was created after the Exxon Valdez ran aground in 1989 and spilled nearly 11 million gallons of crude oil - the worst-ever U.S. spill.

Six fishing vessels plus Coast Guard cutters and salvage vehicles worked to skim the diesel off the water's surface as the tugboat's operator made plans to try again yesterday afternoon to pump the diesel out, she said.

Once the diesel has been removed, the tugboat can be towed back to Valdez by a salvage company that was also helping to skim the sheen off Prince William Sound.

Christensen could not estimate how long the diesel recovery operation would take.

"They no longer see a diesel sheen east of Glacier Island," he said. "There have also been no reports of impacts to wildlife in that area."

It wasn't immediately known how much spilled. Christensen said an estimate could not be done until the fuel was off-loaded to a barge. The Coast Guard said Thursday that two of its tanks - containing about 33,500 gallons of diesel fuel - were damaged.

A dive team inspecting the tug found damage to the hull and a section of keel missing, said Coast Guard Petty Officer Jon-Paul Rios.

The SERVS system provides two escort tugs for each tanker traveling through the sound after leaving the Valdez Marine Terminal with North Slope crude delivered through the trans-Alaska pipeline. Two tankers were due to depart when the accident occurred.

Officials are investigating what caused the grounding. The tug's six crew members tested negative for alcohol use.