BURNS, Oregon (AFP/REUTERS) - The US Federal Bureau of Investigation on Monday sought a peaceful end to the occupation by a self-styled, armed anti-government militia at a US federal wildlife reserve in rural Oregon, as the standoff entered its third day.

The loose-knit band of farmers, ranchers and survivalists - whose action was sparked by the jailing of two ranchers for arson - said they would not rule out violence if authorities stormed the site, although federal officials said they hope to avoid bloodshed.

"The FBI is working with the Harney County Sheriff's Office, Oregon State Police and other local and state law enforcement agencies to bring a peaceful resolution to the situation at the Malheur National Wildlife Refuge," the bureau's Oregon office said in a statement late Sunday.

Officials added that they would not release many details about the siege "due to safety considerations for both those inside the refuge as well as the law enforcement officers involved."

Dozens of protesters are believed to be holed up at the visitor's centre for the reserve.

The demonstrators said they took over the building to show solidarity with ranchers Dwight Hammond, 73, and his son Steven, 46, who were facing imprisonment over fires they set on federal land in the area.

The protesters vowed to prolong the standoff for "as long as it takes" for the court to rescind its arrest order for the two men.

The Hammonds, however, have distanced themselves from the militiamen, and said they were planning to surrender to authorities as ordered on Monday.

"Neither Ammon Bundy nor anyone within his group/organisation speak for the Hammond Family," said a statement from the family's lawyer W Alan Schroeder, which was published in local media.

The local sheriff, David Ward, said the protesters' actual goal was "to overthrow the county and federal government in hopes to spark a movement across the United States".

A Gallup poll released last month shows some Americans view "big government" as the biggest threat to the nation in the future, when asked to choose between that, big labour and big business. The theme has been embraced by the Republican party's contenders for the 2016 presidential nomination, who so far have been relatively silent on the siege.

So far, there has been no visible police presence at the reserve, where several militiamen in vehicles guarded the entrance while others kept watch from a lookout tower.

School was cancelled in the area for the week, and the county courthouse said it would be closed Monday "for security reasons." On the Internet, public opinion was divided about what was quickly dubbed the #Oregonstandoff, with some branding the takeover an act of domestic terrorism.

Critics mocked the group with hashtag labels such as "YallQaeda" and "VanillaISIS". The former refers to the way some Americans from the southern US and rural areas use the word "ya'll (short for "you all") when referring to others, while the latter references 90's rapper Vanilla Ice, who, like the armed militia, is Caucasian.

Protest leader Ammon Bundy is the 40-year-old son of Nevada rancher Cliven Bundy, who was at the centre of an armed, anti-government standoff with authorities in 2014 over grazing rights on public lands.

The younger Bundy, who spoke by phone to CNN on Sunday, called on the government to restore the "people's constitutional rights". He and his brother Ryan, in addition to seeking freedom for the Hammonds, are calling on the federal government to relinquish control of the Malheur reserve.

The Hammonds were convicted of arson after lighting what they said was a controlled fire on their ranch in Harney County that spread, consuming 56 hectares of federal land.

But witnesses at their trial said that Hammond had illegally slaughtered deer on federal property during a hunting expedition and then handed out matches in order to "light up the whole country on fire", according to a Justice Department statement.

Father and son each already have served several months for the offence, but a judge ordered them back to prison to serve the remainder of their five-year sentence, after they lost an appeals court review.

On Monday morning, a group of about a half-dozen occupiers could be seen outside the facility, with some manning a watchtower and others standing around a vehicle they had used to block the road leading to the building. They chatted quietly among themselves as a large group of media looking on. None were visibly armed.

The occupation followed a protest march in Burns, a small city about 80km north of the wildlife refuge, over the imminent imprisonment of the Hammonds.

The FBI declined to give details on plans by federal officials for dealing with the occupiers.

Law enforcement officials and the leader of the occupation, Ammon Bundy, declined to say how many people were occupying the refuge headquarters.

Bundy is the son of Cliven Bundy, a Nevada rancher whose family staged a 2014 armed protest against federal land management officials that ended with authorities backing down, citing safety concerns.

Ammon Bundy told ABC News on Monday that members of his group were armed.

"It's important that we stand and people know that we're serious," he said. "We understand that in order to truly express our First Amendment rights, we have to have our Second Amendment rights."

The First Amendment of the US Constitution protects the right to free speech, and the Second protects the right to bear arms..

The US Fish and Wildlife Service said late Sunday that all of its staff from the facility were safe and accounted for.

Hammond and his son were convicted in 2012 of setting fires which inadvertently spread to public land. On Sunday evening, they travelled to Los Angeles to surrender to federal authorities, according to their lawyer, W Alan Schroeder. They were to be sent back to prison after federal prosecutors won an appeal that resulted in their being re-sentenced to longer terms.

The occupation was part of a decades-old conflict between ranchers and the federal government over Washington's management of hundreds of thousands of acres of range land. Critics of the federal government say it often oversteps its authority and exercises arbitrary power over land use without sufficient accountability.

The Hammond ranch borders on the southern edge of the Oregon refuge, a bird sanctuary in the arid high desert in the eastern part of the state, about 490km south-east of Portland.

The Bundy ranch standoff in 2014 drew hundreds of armed protesters after the Bureau of Land Management sought to seize Bundy's cattle because he refused to pay grazing fees. Federal agents backed down in the face of the large numbers of armed protesters and returned hundreds of cattle.

Malheur National Wildlife Refuge, encompassing 75,630ha, was established in 1908 by President Theodore Roosevelt as a breeding ground for greater sandhill cranes and other native birds. The headquarters compound includes a visitor centre, a museum and the refuge office.

Ironically, Roosevelt was a Republican.