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 company said the pipe had been shut down and the segment where the leak happened had been isolated.
Nearby residents were evacuated, but later allowed to return to their homes.
The accident happened downstream from the famed Yellowstone national park, a major tourist attraction in the US.
Clean-up crews have been deployed to tackle the spill, which was detected early on Saturday.
ExxonMobil spokeswoman Pam Malek told AP news agency an estimated 750 to 1,000 barrels of oil had leaked from the pipe for about a half-hour before it was shut down.
“We recognise the seriousness of this incident and are working hard to address it,” the company said in a statement.
“Our principal focus is on protecting the safety and health of the public and our employees,” it added.
It is unclear how far the slick will travel, but Duane Winslow, the county’s director of disaster and emergency services, said it 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.
There are fears that fish will suffer because of the accident.
“If fish get oil on them, if they break the surface and get oil on them, it tends to plug up their gills and it often is fatal,” said Bob Gobson, of the Billings Fish, Wildlife and Parks Program.
Exxon promised a full investigation into the spill, which occurred in a 12-inch pipeline, running from Silvertip to Billings, downstream from Yellowstone National Park