A South Dakota-based Native American tribe is warning that approving the Keystone oil pipeline would be "an act of war," according to a written statement. 

The statement from the Rosebud Sioux Tribe was posted online following last Friday's House vote in favor of the Keystone pipeline -- and ahead of a vote Tuesday evening in the Senate. 

"We are outraged at the lack of intergovernmental cooperation. We are a sovereign nation and we are not being treated as such. We will close our reservation borders to Keystone XL. Authorizing Keystone XL is an act of war against our people," tribe President Cyril Scott said in the statement

The statement is a reminder that several tribes, in addition to environmentalists, have raised concerns about the pipeline's route -- which would cross tribal land. While the pipeline already is the subject of a lengthy review process at the State Department and of a court battle, it's unclear whether Native American tribes could complicate that process further. 

President Obama has indicated he would like to let the review process play out. But if Keystone supporters corral the 60 votes needed to pass the bill in the Senate, he would have to decide whether to exercise his veto power. 

Even if he does, Republicans have vowed to return to the issue during the new Congress next year.