Federal agents on Wednesday closed in on the last four anti-government militants still holed up at the Malheur National Wildlife Refuge in Oregon after a 40-day-old armed occupation, the FBI said in a statement.
The Federal Bureau of Investigation said no shots have been fired and that negotiations to end the occupation without violence were continuing.
The four protesters were indicted last week along with 12 others previously arrested on charges of conspiring to impede federal officers during the standoff at the compound.
The takeover at Malheur, which began on Jan. 2, was sparked by the return to prison of two Oregon ranchers convicted of setting fires that spread to federal property in the vicinity of the refuge.
The occupation, led by Ammon Bundy, also was directed as a protest against federal control over millions of acres public land in the West.
Bundy and 10 others were arrested in January in Oregon, most of them during a confrontation with the Federal Bureau of Investigation and state police on a snow-covered roadside where a spokesman for the group, Robert "LaVoy" Finicum, was shot dead. A 12th member of the group turned himself in to police in Arizona.
The FBI said its agents moved to contain the remaining four holdouts Wednesday evening after one of the occupiers drove an all-terrain vehicle outside the barricades previously set up by the self-styled militia members at the refuge.
FBI agents attempted to approach the driver, and he sped away back to the compound, after which federal agents "moved to contain the remaining occupiers by placing agents at barricades both immediately ahead of and behind" their encampment, the FBI said.
A number of the occupiers were relating their account of events as they were unfolding via an independent Internet broadcast, "Revolution Radio," that is known to be sympathetic to the occupation.
One woman said FBI agents had moved to within 50 yards (45 meters) of the occupiers' position in the compound. One protester identifying herself as Sandy Anderson reported seeing FBI snipers posted on a nearby hillside with high-beam vehicle lights trained on the compound.
"If they tear gas us, it's the same as firing on us," she said, adding, "Don't come in. Don't do it."
It was not immediately clear how much farther law enforcement officers would go in the latest confrontation. Until Wednesday, FBI and police had largely kept their distance from the buildings occupied by the militants, sealing off access to the refuge headquarters with roadblocks.
However, the standoff has reached a point where it became necessary to take action to ensure everyone's safety, Greg Bretzing, special agent in charge of the FBI in Oregon, said in a statement.
"It has never been the FBI's desire to engage these armed occupiers in any way other than through dialogue, and to that end, the FBI has negotiated with patience and restraint in an effort to resolve the situation peacefully," the statement said.