The scope of Exxon Mobil's oil leak into the Yellowstone River could extend far beyond a 10-mile stretch of the famed waterway, the company has admitted.
As the firm intensified its clean-up of tens of thousands of gallons of spilled crude, Exxon Mobil Pipeline president Gary Pruessing pledged to do "whatever is necessary" to find and mop up oil from the 12in pipeline that broke at the bottom of the river over the weekend.
The company had earlier downplayed assertions from state and US government officials that damage from the spill was spread over dozens of miles, drawing sharp criticism from Montana governor Brian Schweitzer, who planned to tour the damaged areas on Tuesday.
But company officials said their statements were misconstrued and Mr Pruessing promised that crews would begin walking the Yellowstone shoreline as soon as the flooding river receded to look for pooled oil along the banks.
"We're not limiting the scope of our clean-up to the immediate site," he said at a news conference along the river near Laurel, as crews mopped up oil in the background. "We are not trying to suggest in any way that that's the limit of exposure."
Underscoring rising anger over the spill among some river front property owners, Mr Pruessing was confronted after his news conference by a goat farmer and environmental activist who said his partner was made ill by oil fumes and had to be taken to hospital.
"I need to know what we've been exposed to. People are sick now," Mike Scott said. Mr Scott's partner, Alexis Bonogofsky, was diagnosed on Monday with acute hydrocarbon exposure after she experienced dizziness, nausea and trouble breathing, he said.
Mr Pruessing said air and water monitoring had not revealed any health risks but he told Mr Scott the company would provide the public with more information.
The Environmental Protection Agency said officials were still taking air and water samples to determine the impacts.
Exxon Mobil has estimated that up to 1,000 barrels, or 42,000 gallons of crude oil spilled on Friday night before the flow from the damaged pipeline was stopped. The break near Laurel has fouled miles of riverbank, although high water has hindered attempts to find where all the oil went.