An "unprecedented" surge in debris from last year's Japanese tsunami is washing up on Alaska's coastline, environmentalists about to embark on a major cleanup operation said Tuesday.

Floating material including buoys and Styrofoam has washed up on Montague Island, some 120 miles (190 kilometers) southeast of Anchorage, in volumes that clearly suggest a wave of debris from the March 11, 2011 killer tidal wave.

"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, so "it is incredibly difficult to say with complete confidence that a given piece of debris is from the tsunami."

"However, 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," he told AFP, before the start of a planned 12-day cleanup operation, set to start Thursday.

"There is no other possible source for this increase besides the tsunami, so our conclusion is that is where it must be from."

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.

In early April, the US Coast Guard sunk a deserted Japanese trawler that had appeared off the coast of Alaska more than a year after being set adrift by the tsunami.

Also last month, a Japanese schoolboy heard he was getting his ball back, after it was spotted by an observant beachcomber on Middleton Island in the Gulf of Alaska.

Canadian media reported in early May that a Harley-Davidson, with Japanese plates from one of the hardest hit areas, was found by a beachcomber on the Haida Gwaii islands off the coast of British Columbia.