AN ExxonMobil pipeline in the US state of Montana has ruptured, leaking hundreds of barrels of crude oil into the Yellowstone River, officials say.
The leak has caused a 40km plume, fouling the riverbank and forced municipalities and irrigation districts downstream to close intakes.
The break in south-central Montana led to temporary evacuations of hundreds of residents along a 32km stretch. Clean-up crews deployed booms and absorbent material as the plume moved downstream at around 10 km/h.
The river has no dams on its way to its confluence with the Missouri River just across the Montana border in North Dakota. It's unclear how far the plume might travel.
"The parties responsible will restore the Yellowstone River," Montana governor Brian Schweitzer said.
ExxonMobil spokeswoman Pam Malek said the pipe leaked an estimated 750 to 1,000 barrels of oil for about a half-hour before it was shut down. Other Exxon officials estimated up to 158,982 litres of crude oil had escaped.
Duane Winslow, Yellowstone County director of disaster and emergency services, said the plume was dissipating as it moved downstream.
"We're just kind of waiting for it to move on down while Exxon is trying to figure out how to corral this monster," Mr Winslow said.
"The timing couldn't be worse," said Steve Knecht, chief of operations for Montana Disaster and Emergency Services, who added that the plume was measured at 40kms near Pompeys Pillar National Monument.
"With the Yellowstone running at flood stage and all the debris, it makes it dang tough to get out there to do anything."
Brent Peters, the fire chief for the city of Laurel said about 140 people in the Laurel area were evacuated due to concerns about possible explosions and the overpowering fumes. He said they were allowed to return after fumes had decreased.
Mr Winslow said hundreds or residents downstream were told to evacuate in the early morning hours as authorities knocked on doors, but it's unclear how many did.
In a statement, ExxonMobil said it was sending a team to help with clean-up, and that state and federal authorities had been alerted to the spill.
The ExxonMobil Pipeline Company "deeply regrets this release", it said.