AUGUSTA — A Maine environmental group says a proposal by a Canadian oil company could lead to oil from the tar sands of western Canada being piped to Portland.

The Natural Resources Council of Maine says it and four other environmental groups have asked the National Energy Board of Canada to deny Enbridge Inc.’s request to reverse the flow of one of its pipelines in Ontario.

Enbridge says the line will transport oil that isn’t from the Alberta tar sands, but Pete Didisheim of the Resources Council says the request is the first step to bringing tar-sands oil to Portland via pipeline to then be shipped to U.S. refineries.

Critics say tar sands oil requires huge amounts of energy to extract and could cause an ecological disaster in case of a spill.