MONTAGUE ISLAND, Alaska â€"  An "unprecedented" surge in debris from last year's Japanese tsunami is washing up on Alaska's coastline, environmentalists said Tuesday as they embarked on a major cleanup operation.

Floating material including buoys and Styrofoam has washed up on Montague Island, about 120 miles southeast of Anchorage, in volumes that clearly suggest a wave of debris from the 2011 disaster, AFP reported.

The March 11, 2011, tsunami killed more than 15,000 people and washed parts of cities out to sea.

"The debris found on initial surveys of the island showed an absolutely unprecedented amount of buoys, Styrofoam and other high floating debris," said Patrick Chandler of the Center for Alaskan Coastal Studies.

He said debris from Asia has been washing up on Alaska shores for years, but "we have never seen the amount we see now. In the past we would find a few dozen large black buoys, used in Japanese aquaculture, on an outside beach cleanup. Now we see hundreds."

The large-scale clean-up will begin Thursday and take 12 days.

Millions of tonnes of debris are expected to wash up in the coming months and years from the Japanese quake. Researchers in Hawaii have developed computer models to forecast where and when it could come ashore.

A large black float with Japanese lettering, believed to be from the tsunami, was found in January on a beach in Northern California's Humboldt County, The Wall Street Journal reported, with that discovery followed by similar finds by beachgoers in Washington and Oregon.

So far, officials have confirmed tsunami debris in only a few instances. In April, a soccer ball that washed up in Alaska and a Harley-Davidson motorcycle that came ashore in its storage container in British Columbia were both confirmed as having come from the tsunami.

On April 5, an unoccupied fishing trawler that U.S. officials said was cast adrift by the tsunami was sunk by a Coast Guard vessel after it entered Alaskan waters and posed a potential navigation hazard.