Armed cattle ranchers have occupied a national wildlife refuge building in the US state of Oregon, vowing to stay “for years”.

The group took control of the Malheaur National Wildlife Refuge headquarters, approximately 450 kilometres east of the state capital Portland, to protest the jailing of a father and son rancher, Fox News reported.

The protest was reportedly triggered by a judge’s decision to re-jail the two ranchers, Dwight Hammond, 73, and Steven Hammond, 46, for arson. The judge ruled their original sentence was too lenient.

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The Hammond father and son were found guilty of starting a fire on their own property that spread to government land. A court found they started the blaze deliberately to cover up hunting violations.

The decades-old undercurrent of the dispute is reportedly rancher anger at expansion of the wildlife refuge. Photo: AAP

The ranchers were originally sentenced to three months in prison, but an appeal resulted in an additional five year jail term for both father and son, triggering the protest.

After a peaceful demonstration on Sunday (AEDT), a group of protestors seized the building, which serves as a headquarters for US Fish and Wildlife Service employees who oversee the reserve where local wildlife roam free, protected by a federal law dating back to 1908.

“We will be here as long as it takes,” rancher spokesman Ammon Bundy said, according to Fox News.

“We have no intentions of using force upon anyone, [but] if force is used against us, we would defend ourselves.”

The rancher dispute reportedly dates back decades. It was sparked by what the cattle herders saw as coercive behaviour used by the US Fish and Wildlife Service to get ranchers to sell their land in order for the reserve to be expanded, The Last Refuge blog reported.

The protestor spokesman, Ammon Bundy, is the son of Clive Bundy, a Nevada rancher infamous for a standoff with federal agents in 2014 over cattle grazing rights.

Ammon Bundy claimed the group of protestors numbered up to 100.

“We’re planning on staying here for years, absolutely,” he told The Oregonian newspaper.

“This is not a decision we’ve made at the last minute.”

The Hammond father and son, not believed to be in police custody, are reportedly scheduled to voluntarily report to federal prison on Monday to begin their five-year prison sentences. There have been no reports suggesting the pair intend to skip prison.