VANCOUVER -- Concerns south of the border over oil tanker traffic from British Columbia have spurred a U.S. Coast Guard review of proposed increases in Canadian oil exports.

A legislative amendment proposed by Washington state Sen. Maria Cantwell and signed into law by President Barack Obama a couple of weeks ago gives the U.S. marine safety agency six months to conduct a risk assessment of the planned expansion of oil pipeline capacity to the West Coast.

While several proposed projects would see oil from the Alberta oil sands brought to the B.C. coast for export primarily to China, the legislation deals specifically with tanker traffic out of the Vancouver area.

"According to reports, Canada is poised to increase oil tanker traffic through the waters around the San Juan Islands and the Juan de Fuca by up to 300 per cent," said a statement issued by Cantwell's office.

"A supertanker oil spill near our shores would threaten Washington state's thriving coastal economy and thousands of jobs," the Democratic senator said in the statement. "This bill will provide crucial information for Washington coastal communities by requiring a detailed risk analysis...."

The Coast Guard will study the risk of transporting oil via supertanker, tanker and barge through the Salish Sea waterways, which encompasses U.S. and Canadian territorial waters between southern Vancouver Island and the mainland. It includes Juan de Fuca Strait, the Strait of Georgia, Haro and Rosario Straits and Puget Sound.

In order for ships to arrive at port in Vancouver, they usually sail through U.S. waters next to a national marine sanctuary.

The Coast Guard will examine which rules and regulations apply to oil tankers heading to B.C. ports, as well as analyze the toxicity of what is referred to in the legislation as "tar sands" oil -- a derogatory moniker much opposed by the Canadian industry.

Cantwell said the diluted bitumen that will form part of the Canadian oil exports are likely to require special cleanup technology, and the Coast Guard will also assess the spill response capability.

There are two major oil pipeline proposals currently on the table in British Columbia.

Calgary-based Enbridge's Northern Gateway pipeline would transport oil from the Edmonton area to a tanker port in Kitimat, on the north coast.

But the U.S. legislation appears to affect mainly Kinder Morgan's proposal to expand the capacity of its existing oil pipeline from Alberta to the Vancouver area.

A call to TransMountain seeking comment was not returned.

The $4.3-billion TransMountain project would more than double the capacity of the 1,100-kilometre pipeline, from 300,000 barrels a day to 750,000. It would also allow the pipeline, which currently transports crude and refined oil from the Alberta oil sands, to transport diluted bitumen, a heavy oil critics say is more difficult to contain and clean-up in the event of a marine spill.

In operation since 1953, TransMountain runs from just outside Edmonton to Burnaby, and from there the oil is distributed through separate pipelines to local terminals, a refinery and the Westridge marine terminal in Port Metro Vancouver.

The Westridge terminal currently handles about eight vessels a month, five of them tankers. That would increase to about 28 a month, 25 of them tankers, under the proposed expansion plan.

If the application is successful, construction could begin in 2016 and additional oil could be flowing in 2017.

Vancouver city council has passed a motion opposing the expansion, and the mayor of Burnaby -- the site of a 2007 spill from the pipeline -- has spoken out against the project at a public meeting.

A spokesman for Transport Minister Denis Lebel said the two countries are constantly working together for the safe and secure transportation of natural resources across their shared border.

"Our government has been clear: If any project does not meet or surpass our stringent environmental standards, it will not proceed," spokesman James Kelly said in an email.

"Canada has already strengthened our strong record of environmental protection by requiring double-hulled tankers, mandatory pilotage and increasing navigational tools - and that work continues."

According to the Washington Research Council, the state has five major petroleum refineries of its own that process about 560,000 barrels of crude oil a day. Approximately 600 oil tankers and 3,000 oil barges travel through Puget Sound on an annual basis.

The commandant of the U.S. Coast Guard must submit recommendations to the House of Representatives' transport committee by the end of June.