Media playback is unsupported on your device Media caption The BBC's James Cook visits the Standing Rock camps

Hundreds of US military veterans have joined activists in North Dakota protesting against the installation of a multi-billion dollar oil pipeline.

The activists, who are demonstrating in sub-zero temperatures, have been ordered to leave the area by Monday.

It is unclear if they will obey.

The pipeline, which runs close to the Standing Rock Sioux Native American Indian reservation, is nearly complete except for a section running underneath a nearby river.

President-elect Donald Trump has said he supports the completion of the pipeline. He has stocks in Texas-based Energy Transfer Partners, the project's builder, and Phillips 66, which owns one-quarter of the pipeline.

His spokesman says his stance is not related to his investments, but rather is based on his policies. Mr Trump's spokeswoman, Hope Hicks, said she believed he had sold his Energy Transfer Partners stock.

The Sioux and other Native Americans began protesting against the pipeline in April. They say it will run over sacred burial sites and contaminate Standing Rock's water source.

Image copyright Getty Images Image caption Protesters at the site have braved sub-zero temperatures

Image copyright Reuters Image caption Veterans from several military branches have joined the protests

The Dakota Pipeline stand-off explained

Environmental activists and military veterans have since joined the protests. The veterans' group is led by Michael Wood Jr, a former US Marine and Baltimore police officer.

Police deployed at the protests have been accused of beating peaceful protesters.

In a video statement, Mr Wood Jr said: "If the cops there want to be state-sanctioned agents to brutally beat non-violent veterans, that have served their country honourably - if they're going to beat us - then that should be the signal to the rest of the world of what our country's doing."

Image copyright Getty Images Image caption Protesters say a planned oil pipeline will contaminate local water sources

The veterans' group has set up a Facebook page and a crowdfunding campaign to cover food, transport and supplies for new volunteers. The campaign has so far raised more than $870,000 (£680,000).

A post on the Veterans for Standing Rock event page claimed that an additional 2,000 people will protest alongside existing activists. The organisers urged anyone attending to protest peacefully.

They said: "This event (and this event page) will not tolerate hate, violence or divisive behaviour of any kind. We're doing this to support our country so lets do it with honour, working together."

Image copyright Getty Images Image caption Protests are ongoing, despite the bitter cold

The page also urges volunteers to come prepared: "Bring body armour, gas masks, earplugs AND shooting mufflers (we may be facing a sound cannon) but no drugs, alcohol or weapons."

Activists have been demonstrating in the bitter cold, with temperatures set to drop in the coming week. Average temperatures in the area last December were -5C (22F).

North Dakota Governor Jack Dalrymple said on Wednesday it was "probably not feasible" to reroute the pipeline. Mr Dalrymple said he would instead work on rebuilding relations with Sioux leaders. He and the federal government have ordered mandatory evacuations from the protest site.

The local police department said they hope the harsh weather conditions will force people to vacate the camp but there are no plans to remove them by force.