PORTLAND, Oregon (REUTERS) - A group of self-styled militiamen occupied the headquarters of a US wildlife refuge in eastern Oregon in a standoff with authorities, officials and local media reports said on Sunday, in the latest dispute over federal land use in the West.

The occupation, which began on Saturday followed a march in Burns, a small city about 80 km north of the Malheur National Wildlife Refuge, to protest the incarceration of ranchers Dwight Hammond Jr. and his son, Steven Hammond. The two were convicted in 2012 of setting fires on public land to protect their property from wildfires.

A federal court had ordered the Hammonds to be returned to prison after ruling their original sentences were insufficient.

An unknown number of protesters were occupying the refuge's headquarters building in Princeton on Sunday, according to Jason Holm, a spokesman for the Fish and Wildlife Service and the Bureau of Land Management. No employees were in the building, he said in a statement.

Militia leaders said as many as 100 supporters were with them at the refuge, according to the Portland Oregonian newspaper.

Leaders of the occupation include Ammon Bundy, the son of Cliven Bundy, owner of a ranch in Nevada where his family staged an armed protest against the Bureau of Land Management in April 2014, local media reported. The agency had sought to seize Bundy's cattle after he refused to pay grazing fees. Federal agents finally backed down, citing safety concerns, and returned hundreds of cattle to Bundy.

Bundy and his brother Ryan were among of the Hammonds inside the refuge building, according to a report in the Oregonian.

"We want the government to abide by the Constitution ... and to play by the rules," Ammon Bundy told CNN in a phone interview on Sunday. Bundy said some of the occupiers were armed. "We have no intention of using force or being aggressive or going on the offence, but just as all people have the right to defend themselves, that's exactly what that meant - means."

On Saturday, Bundy told the Oregonian, "We're planning on staying here for years, absolutely."

Federal and state authorities have not said how they planned to respond to the occupation.

"We will continue to monitor the situation for additional developments," Fish and Wildlife spokesman Holm said in the statement. He did not immediately return a phone call seeking further details. No one answered a call to the phone number of the refuge.

The refuge is in Harney County in a rural area of southeast Oregon, about 490 km southeast of Portland in the arid high desert of the eastern part of the state.

In a statement issued late on Saturday, Harney County Sheriff Dave Ward said multiple agencies were trying to resolve the issue and he advised caution.

"For the time being, please stay away from that area," Ward said. "Please maintain a peaceful and united front and allow us to work through this situation."

"The facility has been the tool to do all the tyranny that has been placed upon the Hammonds," the Oregonian quoted Ammon Bundy as saying, in a reference to the wildlife refuge's headquarters.

"This is not a decision we've made at the last minute," Bundy added, calling on other militiamen to join them.

Malheur National Wildlife Refuge, encompassing 75,628 hectares, 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.