The double hulled tanker was carrying more than 100,000 barrels of different oil products.
There were no immediate reports of any leak.
"At this time we do not have an estimate as to the amount of product released," refinery owner Tesoro Corp said in a statement.
Chartered by Tesoro, the Seabulk Pride broke free from moorings while being loaded in Nikiski, south-west of Anchorage.
Tesoro said the vessel was cut adrift after being hit by an ice floe.
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.
The Coast Guard said there were no immediate reports of injury.
The Alaska Department of Environmental Conservation sent two tugboats and a helicopter to the scene.
In 1989, the Exxon Valdez supertanker grounded on a reef in Alaska's Prince William Sound, spilling about 50 million litres of crude oil.
The spillage devastated wildlife and took four years to clean up.