The Army Corps of Engineers on Monday said it has finished a review of the disputed Dakota Access pipeline but wants more study and tribal input before deciding whether to allow it to cross under a Missouri River reservoir in North Dakota.

The corps in July gave permission to pipeline developer Energy Transfer Partners, but in September it said more analysis was warranted in the wake of American Indian concerns. The Standing Rock Sioux, whose reservation will be skirted by the $3.8 billion, four-state pipeline, says it threatens its drinking water and cultural sites.

ETP disputes that and said last week it is preparing to bore under the river.

Army Assistant Secretary Jo-Ellen Darcy said in a letter to company officials and tribal Chairman Dave Archambault that "additional discussion with the Standing Rock Sioux Tribe and analysis are warranted." That discussion is to include potential conditions on an easement for the pipeline crossing that would reduce the risk of a spill.

Darcy said in the letter that the Army will work with the tribe on a timeline "that allows for robust discussion and analysis to be completed expeditiously." Army spokeswoman Moira Kelley would not elaborate to The Associated Press on whether a decision would be done by the time President Barack Obama leaves office. Donald Trump, a pipeline supporter, is set to take office in January.

Archambault and ETP spokeswoman Vicki Granado did not immediately respond to requests for comment.

The 1,200-mile pipeline is to carry North Dakota oil through South Dakota and Iowa to a shipping point in Illinois.

The company building the $3.8 billion Dakota Access oil pipeline said before the Army announcement Monday that it expects to finish construction by Dec. 1, except for the small disputed section in North Dakota, and could begin moving crude early next year if the government gives final approval.

In an email to The Associated Press, Dallas-based ETP said it would finish the pipeline within 120 days of getting approval for the easement beneath Lake Oahe, the Missouri River reservoir in southern North Dakota.

Also Monday, officials locked down the North Dakota Capitol after pipeline opponents gathered there, one day before groups planned more than 200 protests at Army Corps of Engineers offices and other sites across the country.

Nearly 470 protesters have been arrested since August supporting the Standing Rock Sioux.

ETP said it has suffered losses "in the millions" to vandalized equipment along the pipeline route in North Dakota. The company said it was taking steps to protect the pipeline from vandalism, but declined to disclose details.

The rallies set for Tuesday at such places as state Army Corps offices, federal buildings and offices of banks that have helped finance the project are seeking to draw Obama's attention.

The groups, including the Indigenous Environmental Network, Honor the Earth and Greenpeace USA, want Obama to permanently halt the construction of the pipeline, the focus of confrontations between police and protesters in North Dakota for months.

A United Nations group that represents indigenous people around the world said the U.S. government appears to be ignoring the treaty rights and human rights of American Indians opposing the pipeline.

The Nov. 4 statement from the U.N. Permanent Forum on Indigenous Issues called on the government to "protect the traditional lands and sacred sites of the Standing Rock Sioux and uphold their human rights commitments."

Forum member Edward John in late October visited a camp in North Dakota that's drawn hundreds of protesters from around the globe. He said he found a "war zone" atmosphere and that "I felt as though I was in an armed conflict zone on foreign soil."

Justice Department spokesman Wyn Horbuckle said the agency has been in communication with law officers, tribal officials and protesters "to facilitate communication, defuse tensions, support peaceful protests, and maintain public safety."