A huge floating dock cast adrift by Japan's killer tsunami has washed up on an Oregon beach, believed to be the biggest pieces of flotsam to make landfall on the US West Coast so far.

The 66-foot (20-meter) long rectangular structure, made of concrete and metal, was spotted floating off the coast on Monday, and then washed in with the high tide on Agate beach, 100 miles (160 kilometers) southwest of Portland.

The Oregon Parks and Recreation Department (OPRD) contacted Japanese diplomats, who confirmed that it was from the March 11, 2011 tsunami, and had drifted 5,500 miles (8,850 kilometers) across the Pacific over the last 15 months.

Hirofumi Murabayashi, deputy consul general at Japan's consulate in Portland, said: "Four floating docks were washed away by the tsunami. This is one of them.

"The other three we don't know where they are, if they're floating somewhere or they sank in the ocean or not," he said.

Confirmation that the dock came from the tsunami came after Japanese writing and markings were found on various parts of the seaweed-covered dock, including "Shibata, Japan" on tires, apparently designed to make it buoyant.

An OPRD spokesman said the dock was bigger than either a trawler scuttled off Alaska in April for safety reasons before reaching land, or a shipping container with a Harley-Davidson inside on a Canada beach at the start of May.

"The container with the motorcycle was a fair bit smaller than this," spokesman Chris Havel said. "Certainly this is the biggest thing that's not a boat that we've had on the Oregon shore in my memory, for the last 17 years."

The dock has been checked for radioactivity - the killer earthquake and tsunami triggered a disaster at the Fukushima nuclear plant on Japan's east coast - but had proved negative, said Havel.

Various debris from the Japanese tsunami have begun washing up on the US and Canadian west coast, and experts predict a surge of flotsam in the coming months.

Japanese officials confirmed that the dock - 66 feet long, 19 feet wide and 7 feet tall - came from the port of Misawa, in Aomori Prefecture in the northern part of Japan. The metal plaque was dated June 2008.

The consulate spokesman said the dock was able to float due to being made of concrete and styrofoam.

"The surface is concrete, there's some steel cables, inside is styrofoam," Murabayashi said. "That's why it floats on the sea. The dock itself doesn't contain any hazardous materials."

The owner of the dock doesn't want it back. "They said they don't wish to have it returned," Murabayashi said. "I believe it will be disposed (of) in Oregon."

Havel said two possible options were being studied: salvage or demolition, and that it may take "one or two days" to decide what to do.

"We're working with some salvage experts to get advice on whether it's even possible to move it, whether we have to wait for some certain conditions like another high tide or whether theres some other way to do it," he said.

KATU 2 television said it had traced a phone number on a Japanese placard to a business located in Tokyo, called Zeniya Marine Services Company, Ltd., which builds docks and other floating marine structures.

Kirk Tite, visiting the Oregon beach with his young son Trevor, said: "It's kind of scary seeing this wash up here, because we all surf.

"If this crossed the Pacific Ocean and it's this big, that means that just about anything of our worst nightmares could cross the Pacific Ocean. So we're kind of frightened of what's to come," he told the broadcaster.