North Dakota's Highway Patrol locked the state capitol building in Bismarck on Monday as more than two dozen people protested the Dakota Access Pipeline, local media reported.
It comes as the U.S. Army Corps of Engineers is expected to make a decision soon on the next steps regarding the pipeline, a spokeswoman said on Monday.
Governor Jack Dalrymple's office, which is located on the capitol's first floor, was not immediately available for comment. The Highway Patrol could not immediately be reached.
The Bismarck Tribune newspaper reported that the building's front doors had been locked as protesters prayed nearby on the lawn.
The $3.7 billion Dakota Access construction project has drawn steady opposition from Native American and environmental activists since the summer.
This past weekend, demonstrators against the oil pipeline near sacred tribal lands briefly blocked two entrances to a work yard. Separately, protesters said a woman was struck by a truck driver who drove over her feet and fired shots in the air.
Protests have at times turned violent, as security dogs attacked activists last summer and more recently as demonstrators set fire to vehicles and equipment.
"We really are concerned about the safety of the people involved in protests. We want to make sure these are peaceful protests. We certainly respect people's First Amendment right," Amy Gaskill, spokeswoman for the Army Corps of Engineers, said on Monday.
Completion of the pipeline, set to run 1,172 miles (1,885 km) from North Dakota to Illinois, was delayed in September so federal authorities could re-examine permits required by the Army Corps of Engineers.
Plans called for the pipeline to pass under Lake Oahe, a federally owned water source, and to skirt the Standing Rock Sioux Reservation by about a half-mile (1 km). Most of the construction has otherwise been completed.
The Standing Rock tribe and environmental activists say the project would threaten water supplies and sacred Native American sites and ultimately contribute to climate change.
Activists are expected to fan out across the United States on Tuesday in renewed protests against the pipeline, which is owned by Texas-based Energy Transfer Partners LP.
(Reporting by Ernest Scheyder in Houston; Editing by Alan Crosby and Matthew Lewis)