ANCHORAGE, Alaska - A tugboat 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 America's worst oil spill.

The Coast Guard said Thursday that the 136-foot (41-meter) 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. The tug reported the grounding in a radio call at 6:15 p.m. local time Wednesday.

Two of the tug's fuel tanks were damaged. The tanks contain an estimated 33,500 gallons (126,800 liters) of diesel fuel, about a quarter of their total capacity.

The Coast Guard said Thursday that it wasn't clear yet how much fuel had spilled but there was a fuel sheen about 3 miles (5 kilometers) long and 30 yards (meters) wide that had drifted away from the vessel. There was no sheen visible around the tug.

Alaska Gov. Governor Sean Parnell had sharp words for Alaska's oil industry, noting that there had been three "significant" recent spills in the Prudehoe Bay oil fields prior to Wednesday's spill in the sound.

"Frankly, when I saw so many spills in such a short time I was indignant that these spills would occur," Parnell said in a statement. "The spills harm both Alaska's environment and Alaska's reputation for responsible resource development. I let the companies know this was not acceptable."

An oil response vessel had arrived and was skimming the water near the diesel sheen on Thursday afternoon.

Chief Petty Officer Dana Warr said some amount of fuel remained in the tug's two tanks, but the amount of remaining fuel was not known. A barge was being brought to the scene to prepare off-loading the fuel.

Lt. Erin Christensen said the process of removing fuel from the tanks would begin Thursday night and was expected to take about eight hours. She said an estimate on how much fuel was spilled would have to wait until the remaining diesel was off-loaded.

Coast Guard Petty Office Jon-Paul Rios said the tug's crew deployed 200 feet (60 meters) of fuel containment booms around the vessel after clearing the reef and continuing to deeper waters.

The crew and Coast Guard acted quickly, he 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 4-to-5 foot (1.5 meter) section of the keel also was missing. A sheen of fuel was visible on the water, Rios said.

The tug, called the Pathfinder, is part of the Ship Escort Response Vessel System - the system that was put in place following the 1989 Exxon Valdez disaster when the 987-foot (300-meter) tanker ran aground on the same reef and spilled nearly 11 million gallons (41 million liters) of crude oil into Prince William Sound.

The SERVS system provides two escort tugboats for each tanker traveling 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 very different from the Exxon Valdez accident in which an enormous amount of black crude oil spilled into Prince William Sound.

The spills in the Prudehoe fields included one discovered Monday that contaminated several thousand square feet of snow-covered land. Last month, a spill 46,000 gallons (174,000 liters) of oil, water and natural gas was reported. And officials also investigated another oil spill of about 7,100 gallons (26,900 liters) of water with oily residue.