Gov. Brian Schweitzer said Saturday the oil giant and any other responsible parties would pay for the cleanup.
Schweitzer said in a release that Montana Disaster and Emergency Services and other state agencies were notified by Yellowstone County officials that oil was released into the river Friday night from a 12-inch-diameter ExxonMobil Co. pipeline at Laurel, Billings' KTVQ-TV reported. The spill started about 11:30 p.m. and the slick had spread more than 150 miles down river to Miles City by Saturday afternoon, The Billings Gazette reported.
People in the immediate vicinity were evacuated for a time due to the odor from the oil.
"I still smell like oil," said Glenn Wells, who was among those evacuated. "My whole house smells like diesel fuel. It was everywhere on the river -- an oil slick on Billings' West End."
Oil could be seen collecting in eddies and clinging to plant life and the riverbanks, the Gazette said.
"Disaster and Emergency Services, Department of Environmental Quality and other involved state agencies will monitor Exxon Mobile and any other responsible parties until this spill and impacts of this spill are completely cleaned up," the governor said. "The parties responsible will restore the Yellowstone River."
The TV station said Exxon Mobile said an "undetermined amount of crude oil" had spilled into the Yellowstone.
"We regret the release," the Gazette quoted ExxonMobil spokeswoman Pam Male as saying. "It's important that we get it resolved."
The Gazette said ExxonMobil had two cleanup divisions in place along the river and was expecting 100 more contractors from Washington to help in the effort. The crews were using absorbent pads and booms to leach up oil from the riverbanks, the Gazette said.
"Exxon really is going to be the incident command," Duane Winslow, Yellowstone County director of disaster and emergency services, told the newspaper.
"It's going to be a heck of a cleanup."