"Our captain of the port is on scene and he says that the hull is intact. So there's no product leaking from the vessel at this time," Coast Guard Petty Officer Gail Dale said.

The double-hulled tanker, the Seabulk Pride, was loading heavy vacuum gas oil and unleaded gasoline from the refinery in Nikiski, Alaska, on the Cook Inlet, refinery owner Tesoro Corp. said in a statement.

Ice conditions at the time of the accident were "extreme," with a lot of ice floating in the water, said Lynda Giguere of the Alaska Department of Environmental Conservation.

When the mooring line broke, about 200 gallons (757 liters) of oil being loaded into the tanker fell onto the ship's deck and 10 gallons (38 liters) spilled into the water, the Coast Guard said.

The tanker is owned by Seabulk International Co., a subsidiary of Seacor Holdings Inc. and leased by Tesoro.

Coast Guard Petty Office Thomas McKenzie, based in Juneau, Alaska, said it was unclear whether other shipments in the area would be disrupted because of the grounding. The Coast Guard said there were no immediate reports of injury.

The tanker, which is also double-bottomed, was carrying nearly 4 million gallons (15 million liters) of Vacuum Tower Bottom Blend, a residual, asphalt-like oil that is not refined at Tesoro's Nikiski facility.

It was also carrying nearly a million gallons (3.8 million liters) of bunker oil, gasoline, heavy vacuum gas oil and diesel fuel, the Alaska Department of Environmental Conservation said.

Tesoro said its loading lines and docks were secure.

In 1989, the Exxon Valdez supertanker grounded on a reef in Alaska's Prince William Sound, spilling about 11 million gallons (42 million liters) of crude oil.

Shares of Tesoro, based in San Antonio, Texas, fell $3.11 to $66.90 and Seacor Holdings' stock fell $2.32 to $71.12 in afternoon New York Stock Exchange trade.