Three environmental groups and the White Earth Band of Ojibwe have asked state regulators to reconsider its decision on a proposed crude oil pipeline across northern Minnesota, arguing that environmental risks and tribal rights haven’t been fully considered.

Requests by the band, Friends of the Headwaters, Honor the Earth, and the Carlton County Land Stewards set the stage for potential litigation if the Minnesota Public Utilities Commission declines to reconsider its recent approval of the need for Enbridge Energy’s Sandpiper ­pipeline.

“The public has really tried to be involved in these proceedings, and they have not yet been heard,” said Kathryn Hoffman, an attorney representing Friends of the Headwaters, a Park Rapids-based group that objects to building pipelines through the upper Mississippi River watershed. “I am hopeful that the commission will still take the time to hear the public, to consider different alternatives and think about what’s best for Minnesota.”

In June, the PUC voted 5-0 to approve a certificate of need for the $2.6 billion, 610-mile pipeline from North Dakota to Superior, Wis. The vote came after the commission chairwoman and one commissioner expressed worries about the time and expense for state agencies to conduct full environmental studies of completely rerouting the line around the headwaters and lakes regions.

Environmental and tribal attorneys, in separate petitions filed over the past five days, said regulators had a duty under state law or historic treaties with Indian tribes to conduct a full environmental review of alternatives, with the expense charged to Calgary-based Enbridge Energy.

Joseph Plummer, an attorney for the White Earth Band, said the Sandpiper route and a second pipeline proposed on the same corridor, will run through wetlands and watersheds feeding some of the most important wild rice lakes in Minnesota. The route passes through Clearbrook, then goes south toward Park Rapids, and finally east to Superior.

Enbridge’s preferred route, which largely follows existing pipelines and transmission lines, is undergoing a state environmental review. A partial reroute opposed by environmental and tribal groups also is being studied. That route would go through the Mississippi headwaters, but avoid lakes and wetlands east of Park Rapids.

State regulators are not obligated to reconsider their action. However, if pipeline opponents hadn’t asked for reconsideration, they would have lost their right to appeal the PUC’s decision in court. Honor the Earth, an environmental group led by White Earth activist Winona LaDuke, has said it plans an appeal. The other groups said litigation remains an option.

Two environmental groups earlier challenged the PUC’s review procedures, and are awaiting a ruling from the Minnesota Court of Appeals. Attorneys for the band and Honor the Earth say their demands for environmental studies are tied to a broader effort to gain state recognition of tribes’ right to gather wild rice from off-reservation lakes under 19th century treaties.

Treaty rights activists are planning to harvest rice without state permits at a Nisswa lake later this week in a confrontation with the state Department of Natural Resources. Frank Bibeau, an attorney for Honor the Earth, said the tribes have a treaty-based interest in lands crossed by the pipeline.

“What I would liken it to is a federal conservation easement across the entire territory north of I-94,” he said.

Enbridge spokeswoman Lorraine Little said the company anticipated the requests to reconsider.

“For the most part, the parties raised issues that were discussed during the public hearings and fully briefed and considered by the commission before their unanimous decision to grant the permit,” Little said by e-mail. The company will file a formal response later, she said.

The Sandpiper project and its companion pipeline known as Line 3 on the same proposed route will undergo a state environmental study that is the “equivalent to a full Environmental Impact Statement — it will require the same amount of thorough, scientific inquiry,” she added.

In the first stage of that review, state officials are wrapping up a series of public meetings this week in communities across northern Minnesota. The last meetings will be held Wednesday and Thursday in Carlton and McGregor.

The Sandpiper pipeline would carry North Dakota crude oil to refineries in the Midwest and elsewhere. Line 3 replaces an existing line from Alberta’s oil fields.