A county sheriff and judge have demanded that self-styled militiamen occupying a remote US wildlife centre peacefully end their three-day-old anti-government siege.

They've told the protesters: "It's time for you to leave our community."

The flashpoint for Saturday's takeover of the headquarters of the Malheur National Wildlife Refuge outside the town of Burns, Oregon, was the imminent incarceration of two ranchers convicted of arson and re-sentenced to longer prison terms.

But the occupation marked the latest flare-up of anger against the US government over federal management of public land in the West, long seen by political conservatives in the region as an intrusion on individual freedom and property rights.

Federal authorities have so far kept their distance from the wildlife refuge, which remained closed to visitors.

The FBI said in a statement it was seeking a "peaceful resolution to the situation", while Obama administration officials said US law enforcement officers had been told to avoid a violent confrontation with the occupiers.

Protest leader Ammon Bundy, whose father's ranch in Nevada was the scene of an armed stand-off against federal land managers in 2014, told reporters on Monday his group had named itself Citizens for Constitutional Freedom and was making a stand for personal liberty.

"They (the federal government) are coming down into the states and taking over the land and the resources, putting the people into duress, putting the people into poverty," he said.

Flanked by supporters, Bundy declined to say how many were participating in the takeover. But about a half-dozen occupiers were visible to reporters, some in a watchtower on the property and others standing around a vehicle used to block an access road.

The two ranchers whose cause Bundy's group has embraced - Dwight Hammond Jr, and his son, Steven - turned themselves over to federal authorities in California earlier on Monday.

Some residents in Burns, a town of some 3000 people about 451 kilometres southeast of Portland, voiced sympathy with the militia group's cause, if not its methods.

But many said they viewed the occupation as mostly, if not entirely, the work of outside agitators - a sentiment echoed by Harney County Sheriff David Ward.

"You said you were here to help the citizens of Harney County," Ward said at a news conference in Burns, addressing Bundy's group in a statement he read on behalf of himself and county Judge Steven Grasty.

"It is time for you to leave our community, go home to your families, and end this peacefully."