Opponents of the Dakota Access oil pipeline in the US admit they are running out of legal options to block the controversial project after the builders won federal permission to tunnel under the Missouri River.

The US Army said on Wednesday it had granted the final permit for the pipeline following President Trump’s order to get it going.

Native American tribes and climate activists have vowed to fight the pipeline, declaring it will desecrate sacred sites and endanger drinking water.

“We’re running out of options, but that doesn’t mean that it’s over,” David Archambault II, chairman of the Standing Rock Sioux Tribe, told Reuters. “We’re still going to continue to look at all legal options available to us.”

The pipeline, nearly 2,000 kilometres long, will move crude from North Dakota oilfields to Illinois on the way to US refineries in the Gulf of Mexico.

The project has drawn thousands of protesters to the plains of North Dakota. Camps have been set up near the site and there’ve been several violent clashes with security forces and hundreds of arrests.

President Obama’s administration delayed its completion pending a review and ordered an environmental study.

But within days of taking office Donald Trump issued orders to expedite the Dakota Access Pipeline (DAPL) and to revive another major project, Keystone XL.

Legal experts agree that Dakota’s opponents face long odds in convincing any court to halt the $3.8 billion project led by Energy Transfer Partners LP, which could now begin operations as soon as June.