The US Coast Guard Thursday fired on a Japanese "ghost ship" in a bid to sink it after it floated into waters off Alaska after last year's tsunami in Japan, presenting a danger to shipping.

The deserted trawler was first spotted off the coast of Canada on March 24, having drifted thousands of miles (kilometers) across the Pacific Ocean on the current following the March 2011 tsunami.

"The reason for sinking the vessel is that it's currently a significant hazard to navigation," Sara Francis, a spokeswoman for the Alaska Coast Guard, told AFP.

"It's between 150 and 200 feet long, it's unmanned and unlit and adrift. In the dark, it presents a significant danger to other vessels transiting that area."

The Japanese owners of the ship had said they did not want it back. Early Thursday, a Canadian crew had tried to save it, but after a closer inspection they abandoned the idea and the operation to sink the ship began.

"Right now, we've begun our initial salvo," said Kip Wadlow, Coast Guard chief petty officer in the Alaskan capital of Juneau.

"Once that initial salvo concludes, there's going to be a hour to two hours where the crew assesses how effective the first salvo was and then, we'll just take it from there."

He added that if a second salvo was needed to scuttle the ship completely, the Coast Guard would have time to prepare and would then assess the results.

They would "continue like that until the vessel is successfully sunk," Wadlow said.

The Coast Guard also offered assurances that there was no risk of pollution from the ship which could threaten the pristine Alaskan and Canadian coasts.

"If there is any fuel on board it's expected to be diesel fuel, which will disappear very quickly and pose limited risk to the environment," said Francis.

The boat was the largest piece of debris so far to wash up on the US coast after the March 11, 2011 tsunami.

Some 20 million tons of debris have been washed up off the Japanese coast and researchers in Hawaii have developed computer models to forecast its movement and predict where and when it could come ashore.