The FBI has moved in on the last four occupiers of a wildlife reserve in Oregon who are protesting against federal control of local land.
Agents have been placed behind barricades near the self-styled militia’s encampment, a statement said.
The four have refused to leave despite the arrest of the group’s leader Ammon Bundy last month. He has urged those remaining to stand down.
The Malheur National Wildlife Refuge was seized early in January.
The armed takeover was sparked by the return to prison of two Oregon ranchers accused of burning federal land.
It developed into a wider protest demanding the return of government-controlled land to locals.
Mr Bundy and others were arrested late in January in a confrontation with police that left one of the activists, LaVoy Finicum, dead.
As events unfolded some of the occupiers have been broadcasting live online.
A Republican politician sympathetic to the group, Michele Fiore, has been speaking to them, trying to calm those inside.
‘Remember Braveheart?’ By James Cook, BBC News
The live stream of events inside the refuge has been a strange mixture of prayer and paranoia, of fear and defiance.
The occupiers have repeatedly accused the federal government of persecuting them and they have attacked President Barack Obama for trying to “take away guns from people who are depressed”.
With tens of thousands listening, the militia revealed that they had been eating well, talking about hearty cooked breakfasts, pork fried rice and steaks.
At one point, an occupier compared himself to the Scottish patriot William Wallace, suggesting that he too would rather die than give up his freedom.
“Remember Braveheart? Braveheart took it to the end even when he was tortured,” he said.
At other times members of the quartet have sounded despairing, repeatedly expressing the fear that they will be shot dead.
They have been discussing giving up in the morning but they want assurances that they will not be prosecuted.
Those still inside the refuge have been named as David Fry, 27; Jeff Banta, 46; and married couple Sean Anderson, 48, and Sandy Anderson, 47.
The FBI said they moved in on the four after one of them drove a vehicle outside barricades erected by the group.
“We reached a point where it became necessary to take action in a way that best ensured the safety of those on the refuge, the law enforcement officers who are on scene, and the people of Harney Count,” the statement said.
No shots have been fired and the FBI said negotiations to end the stand-off were under way.
The Oregon stand-off
How did it begin?
In October, a federal judge ruled the sentences on two Oregon ranchers, Dwight and Steven Hammond, for burning federal land were too short and jailed them for about four years each.
Angered by the ruling, Nevada native Ammon Bundy began a social media campaign backing them and travelled to Burns, Oregon, organising meetings.
His group attracted supporters from across a number of states and Mr Bundy called it Citizens for Constitutional Freedom. On 2 January the armed militiamen took over the Malheur National Wildlife Refuge – and widened the range of demands.
What are the militia’s aims?
It is an extension of the Sagebrush Revolution of the 1970s and 1980s that demanded the transfer of federal land in many western states to local control.
Mr Bundy’s own father – a Nevada rancher – had been involved in a protest over cattle-grazing rights in 2014. One policy is to try to persuade ranchers to tear up their federal grazing contracts.
Although many local residents are sympathetic with its cause, many also oppose the occupation of the refuge. Even the local ranchers who are serving the longer sentences distanced themselves from the militia.
Are militias legal?
The term has a complex history and generally refers to those outside the official military who can be called on in times of need. The US Constitution refers to the president having command of “militia of several states” and that Congress “can call forth militia” to tackle insurrection and invasion.
Those who form such militias cite the constitution and various references in federal and state law as granting them legality.