Oil swirls in a flooded gravel pit in Lockwood, Montana. The Exxon Mobil pipeline that runs under the Yellowstone River near Billings, in south-central Montana, ruptured and dumped an unknown amount of oil into the waterway, prompting temporary evacuations along the river.

Washington - An oil spill that leaked as much as 42 000 gallons of crude into the Yellowstone River near Billings, Montana, had officials from Exxon Mobil and the government scrambling on Sunday to assess damage and pinpoint the cause of the leak.

Exxon apologised to those affected by the leak - which the company said was shut off within minutes of its discovery early Saturday - and Montana Governor Brian Schweitzer said state agencies would ensure the spill is cleaned up.

“The parties responsible will restore the Yellowstone River,” Schweitzer said in a statement.

Air monitoring has detected no threat to residents from the spill, which fouled river water over about a 10-mile stretch between Billings, the state's largest city, and the nearby town of Laurel, Gary Pruessing, president of Exxon Mobil Pipeline Company, told reporters.

Municipal water systems also have shown no signs of infiltration by the oil, he said.

The company said it deployed airplanes to fly over the spill and monitor its spread, and it planned further flights.

Exxon discovered the leak when a loss of pressure was detected on the pipeline, and within six minutes the line was shut off, Pruessing said.

Some residents were briefly evacuated on Saturday as a precaution, but they have returned home, officials said.

“The turbulence of the river is going to break (the oil) apart and move it in a lot of different directions,” he said.

Exxon would have about 100 people working on the oil spill by the end of Sunday, he said. The company has deployed oil-absorbent pads and boom to aid in cleanup efforts.

The cause of the leak, which happened on a pipeline that dates from 1991, remains unclear. Exxon estimated the oil release at up to 1,000 barrels, which equals 42 000 gallons.

“We had no indications that there were any issues with this pipeline,” Pruessing.

Pruessing said the pipeline had been shut in May as a precaution as the river approached a high-water point, but that it was judged to be safe and turned back on.

The spill occurred on a pipeline that runs only in Montana, from Silver Tip to Billings, where the crude is refined.

The US Environmental Protection Agency and the Montana Department of Environmental Quality were among the agencies dealing with the leak.

An initial assessment showed that no wildlife was harmed, Pruessing said.

“We will stay with the cleanup until it is complete, and we sincerely apologise to the people of Montana for any inconvenience the incident is creating,” Pruessing said.

The leak came one day after a Maryland jury awarded plaintiffs suing Exxon $1.5-billion in damages for a 2006 leak at a gasoline station. - Reuters