US president Barack Obama has said army engineers are examining whether the Dakota Access oil pipeline can be rerouted in southern North Dakota to alleviate the concerns of Native Americans.

Mr Obama said his administration is monitoring the situation closely but will "let it play out for several more weeks".

The Standing Rock Sioux tribe claims that the 3.8 billion dollar (£3.1 billion) pipeline from North Dakota to Illinois will threaten its drinking water and destroy sacred sites.

Protests which have included clashes with police have gone on for several months in North Dakota.

Mr Obama called it "a challenging situation", saying there is an obligation for protesters to be peaceful and for authorities to show restraint.

The 1,200-mile pipeline is largely complete outside of North Dakota.

Meanwhile, documents have revealed that Native American artefacts found last month in North Dakota along the route of the pipeline were not reported to state regulators for 10 days.

Public service commissioner Julie Fedorchak said she is disappointed regulators were not notified earlier. The matter is to be discussed during a commission meeting later.

North Dakota chief archaeologist Paul Picha said he was notified of the discovery in a timely manner but did not report it to the commission because he thought the pipeline company would. He added that the site was properly handled.

In an October 27 letter, Texas-based Energy Transfer Partners detailed the finding of stone cairns and other artefacts.

Consultants for the pipeline company determined there was "a low likelihood" of buried artefacts, but recommended avoiding the site.