ELLSWORTH, Maine — Maine could become another front in the high-stakes economic and environmental battle over oil from Canadian tar sands that is dividing communities north of the border and has prompted a two-week civil disobedience protest at the White House.

Five environmental organizations, including one here in Maine, are seeking to block a Canadian pipeline proposal that they claim is the first phase in a larger project to allow oil derived from tar sands fields in Alberta to flow to U.S. markets through Portland.

The groups, all of which are critical of oil extraction from Canadian tar sands because of the environmental impacts, are suggesting that global oil shipper Enbridge is disguising the true scope of the company’s goals and attempting to avoid public scrutiny.

They are urging the Canadian National Energy Board to order a more comprehensive look at the proposal while simultaneously trying to raise alarm about an environmental issue garnering international headlines thanks to another pipeline proposed through America’s heartland.

“We want the public and regulators to know that this is potentially coming our way,” said Pete Didisheim with the Natural Resources Council of Maine.

The council joined the Vermont Natural Resources Council and the Canadian organizations Environmental Defense, Equiterre and Pembina Institute in filing a complaint with the National Energy Board.

Enbridge, a Calgary-based company that is among the world’s largest movers of oil, is seeking regulatory approval in Canada to reverse the direction of oil flow in a pipeline connecting the towns of Sarnia and Westover in Ontario. Oil in the pipeline, known as Line 9, now flows westward but the company wants to enable oil shipments to refineries in the east.

Enbridge spokesman Glenn Herchak wrote in an email on Wednesday that the reversal project is a standalone project that “has a unique commercial demand and is not contingent on any other project.”

But the groups contend this is the first step toward reviving Enbridge’s 2008 project, known as “Trailbreaker,” that aimed to ship oil from the tar sands regions in Alberta along those same pipelines to the east, with an eventual goal of reaching the oil shipping terminals in Portland. That project was dropped because of market conditions.

To do this, Enbridge would need to reverse oil flow between Westover, Ontario, and Montreal as well as between Montreal and Portland.

Dave Cyr, treasurer of the Portland Montreal Pipe Line Co., said Wednesday those discussions have resumed, although it is still too early to say what will come of the negotiations.

“We are interested in reversing the flow in one of our pipelines, allowing us to move western Canadian crude to the East Coast,” Cyr said. He added that, at present, the company is only using roughly one-third of its oil capacity.

Herchak wrote that the company is monitoring the markets and customer demand.

“Should the market conditions indicate that the reversal of the Westover-to-Montreal section of Line 9 be further developed, Enbridge would file an application with the NEB for such a project,” he wrote.

Alberta’s tar sands oil fields have, in recent months, become a top target for environmental groups on both sides of the border. Critics claim extracting oil from tar sands destroys vast areas of boreal forest, causes extensive air and water pollution and releases massive amounts of climate-warming greenhouse gases into the atmosphere.

“We should be making every effort to prevent Portland from becoming the tar sands capital of the eastern United States,” Didisheim said in a statement. “This is one of the dirtiest fossil fuels on the planet, causing massive environmental impacts across a widening expanse in western Canada.”

Supporters of the energy projects, meanwhile, insist that buying oil from Canada is preferable to sourcing it from many of the politically troubled oil-rich nations elsewhere around the world and say the extraction and pipeline projects generate thousands of jobs.

The tar sands issue has received widespread media coverage in recent months as federal regulators in the U.S. consider a proposal by TransCanada Corp. for a 1,700-mile pipeline that would carry crude from Alberta to refineries in Texas.

The company and project supporters claim construction of the pipeline will create up to 20,000 jobs. But hundreds of opponents of the Keystone XL project have been arrested in recent weeks while waging daily sit-ins outside of the White House.

In their letter to the Canadian regulators, the five groups fighting the Enbridge proposal raised concerns about the potential for spills from the pipeline because of what they said was the more corrosive nature of tar sands oil.

Didisheim pointed out that the Portland-to-Montreal pipeline passes near Sebago Lake — a key water source for the Portland region — and over the Androscoggin River.

But Cyr with Portland Montreal Pipe Line Co. said he believes the crude now carried through the company’s pipeline is similar to the crude that would come from western Canada. And he stands by the safety record of his 70-year-old company, which has moved more than 4 billion barrels of crude oil to Canada since the first pipeline opened in 1941.

“We are a pipeline operator and we are good at what we do,” Cyr said. “We can move western Canadian crude safely through our system.”

As for as the political and environmental discussion over tar sands oil, Cyr said he would leave that to others to debate.