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NEW YORK (Reuters) - A pipeline operated by Exxon Mobil Corp leaked as many as 1,000 barrels of crude oil into the Yellowstone River in Montana and has been shut down, the company said.
Exxon officials said on Sunday there were traces of oil up to 10 miles downriver from the site of what they called a "very unusual" event, though the governor of Montana said the spill may have spread further.
The U.S. oil company said it had slowed processing rates at its Billings refinery following the leak but did not expect supply disruptions in the area.
Exxon found the leak from the EMPCo pipeline early on Saturday morning. The pipeline runs only in Montana, from Silver Tip to Billings.
The cause of the leak remains unclear. Exxon estimated the oil release at anywhere from 750 to 1,000 barrels. One barrel of oil is equivalent to 42 gallons (163 liters), and the pipeline typically transports 40,000 barrels a day.
"We had no indications that there were any issues with this pipeline," Gary Pruessing, the president of EMPCo, told reporters on a conference call on Sunday.
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.
Montana Governor Brian Schweitzer questioned Exxon's contention that the spill into the Yellowstone, the longest undammed river in the United States, was concentrated within a 10-mile area.
"This is a lot of wild country, and they haven't any idea whether it's 5 miles, 50 miles or 100 miles, they're guessing," Schweitzer, a Democrat, told Reuters in a telephone interview.
A full assessment of the oil's spread will not be possible until small boats can be deployed in the river, he said.
Exxon said earlier that the river's turbulence made it unsafe to use boats, but that the company had sent aircraft to view the spill.
Exxon discovered the leak when a loss of pressure was detected on the pipeline, which dates from 1991, and within six minutes the line was shut off, company officials said.
Some residents were briefly evacuated as a precaution but were allowed to return home later.
Exxon said it would have about 100 people working on the spill by the end of Sunday.
Alan Jeffers, an Exxon spokesman, said in response to Schweitzer's comments that the company made its assessment that the spill appeared to be confined to within a 10-mile area based on what it had observed and what was reported to it.
"The bulk of it appears to be in that area, but we're continuing to look for oil along the river," he said.
Schweitzer said he was concerned about damage from the spill to farm fields recently flooded when the Yellowstone overflowed, and to the state's trout fishing industry.
Air monitoring has detected no threat to residents from the spill, EMPCo's Pruessing said. Municipal water systems also have shown no signs of infiltration by the oil, he said.
The Billings Gazette on Sunday cited conflicting reports from Exxon officials as to the extent of the leak. The paper carried pictures of absorbent bags piled along the side of a road running parallel to the river, ribbons of oil sticking to logs and one of a turtle stuck in oily mud.
Environmental cleanup firm Clean Harbors is on-site assisting with the early remediation efforts, Exxon 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.
Additional reporting by Emilie Ritter, Alex Dobuzinskis and Laura Zuckerman; Editing by Dale Hudson and Maureen Bavdek