An oil pipeline rupture beneath the Yellowstone River has prompted temporary evacuation of residents in south-central Montana, authorities said on Saturday.

Oil slick from the rupture fouled more than 40 km of the waterway, local newspaper The Billings Gazette reported, quoting state officials.

Brent Peters, the fire chief for Laurel, a town about 1.5 km downstream from the rupture, said the spill from the 30cm diameter pipe owned by ExxonMobil occurred some time around 11:30 p.m. (1830 GMT) Friday.

About 140 people in Laurel were evacuated early Saturday due to concerns of possible explosions and the overpowering fumes, and then allowed to return at about 4 a.m. (1100 GMT) after fumes had dissipated, Peters said.

He also mentioned that the river where the leak occurred was 230 meters wide, and the oil slick appeared to be about six meters wide.

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," Winslow said, adding that hundreds of residents downstream have been told to evacuate, but how many did remains unclear.

Exxon has not said what caused the leak, but Yellowstone County officials said the rupture might be caused by debris, which was made possible by high waters eroding parts of the river bed and exposing the pipeline.

Exxon spokeswoman Pam Malek said the pipe leaked an estimated 750 barrels to 1,000 barrels of oil for about half an hour before it was shut down.

"It's going to be a heck of a cleanup," Winslow said.

Exxon has been dealing with the spill by calling in its global response team for emergency cleanup. The local refinery's cleanup team has been working most of Friday night and throughout Saturday.

"We regret the release. It's important that we get it resolved," Malek said.

The cleanup effort would be monitored by the state Department of Environmental Quality, Department of Fish, Wildlife and Parks and the Environmental Protection Agency.