LAUREL, Mont. - Exxon Mobil Corp. acknowledged under political pressure Monday that the scope of its pipeline leak into the Yellowstone River could extend far beyond a 10-mile stretch of the famed waterway.

As the company intensified its cleanup of tens of thousands of gallons of spilled crude, Exxon Mobil Pipeline Co. President Gary Pruessing pledged to do "whatever is necessary" to find and mop up oil from the 12-inch pipeline that broke at the bottom of the river over the weekend.

The company earlier had dismissed assertions from state and federal officials that the spill was spread over dozens of miles. That drew sharp criticism from Montana Gov. Brian Schweitzer.

On Monday, Pruessing pledged that crews would begin walking the Yellowstone shoreline as soon as the flooding river recedes to look for pooled oil along the banks.

"We're not limiting the scope of our cleanup to the immediate site," Pruessing 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 riverfront property owners, Pruessing was confronted after his news conference by a goat farmer and environmental activist who said his partner was sickened by oil fumes and had to be taken to the emergency room.

"I need to know what we've been exposed to. People are sick now," Mike Scott said. Scott's partner, Alexis Bonogofsky, was diagnosed Monday with acute hydrocarbon exposure after she experienced dizziness, nausea and trouble breathing, he said.

Pruessing said air and water monitoring had not revealed any health risks. But he told Scott the company would provide the public with more information.

Exxon Mobil has estimated that up to 1,000 barrels, or 42,000 gallons, of crude oil spilled Friday night before the flow from the damaged pipeline was stopped. The break near Laurel has fouled miles of riverbank, although high water has hobbled attempts to find where all the oil went.

Montana's Disaster and Emergency Services Division was unable to give an update on the spill Monday, and Environmental Protection Agency officials also could not immediately provide any new information.

The 20-year-old Silvertip pipeline delivered 40,000 barrels a day to a refinery in Billings along a route that passes beneath the river.

The line was temporarily shut down in May after Laurel officials raised concerns that it could be at risk as the Yellowstone started to rise. And regulators twice in the last year warned Exxon Mobil of several safety violations along the line.

The company decided to restart the line after examining its safety record and deciding it was safe, Pruessing said.

The U.S. Department of Transportation, which oversees pipelines, notified Exxon Mobil in July of seven potential safety violations and other problems along the pipeline. Two of the warnings faulted the company for its emergency response and pipeline corrosion training.

Transportation Department spokeswoman Patricia Klinger said the company has since responded to the warnings and the case was closed.

The company also was cited for "probable violations" in a February letter. Those included inadequate pipeline markers in a housing development, a section of pipeline over a ditch covered with potentially damaging material and debris, vegetation in a housing area that covered a portion of line and prevented aerial inspections, and a line over a canal not properly protected against corrosion.

Despite Exxon Mobil's pledge to see the cleanup through to its end, the Yellowstone spill has amplified calls from some safety advocates and environmentalists who want the government to impose more stringent regulations on the industry.

It follows other high-profile pipeline accidents in recent months. A Pacific Gas & Electric Co. natural gas line exploded in San Bruno, Calif., in September, killing eight people, and an Enbridge Energy pipeline failure in July 2010 spilled 900,000 gallons of crude into Michigan's Kalamazoo River.

Anthony Swift, a policy analyst with the Natural Resources Defense Council, said the fact that Exxon Mobil's Silvertip line was apparently in compliance with federal rules underscores that those rules need to be strengthened.

"These are the sort of spills that we shouldn't be tolerating," Swift said. "We need to incorporate tougher safety standards."

The cause of the Montana spill has not yet been determined. Company and government officials have speculated that high waters in recent weeks may have scoured the river bottom and exposed the pipeline to debris that could have damaged the pipe.

The company said only one case of wildlife damage - a dead duck - had been reported, but Pruessing said that could not be confirmed. A local newspaper, the Billings Gazette, has run pictures of a turtle and a group of pelicans apparently with oil on them.

A six-person team from International Bird Rescue was in Montana to help with any wildlife rescues that might arise. U.S. Fish and Wildlife personnel on the scene referred questions about wildlife impacts to an Exxon Mobil spokesman.

Initial progress on the cleanup will be tested in coming days as the Yellowstone starts to rise again, making it harder to get to oil-soaked areas and possibly leading to cleaned areas becoming fouled again.

The Yellowstone River at Billings had dropped nearly 2 feet by Monday from its peak Saturday morning, according to the National Weather Service. But temperatures reached the mid-90s Sunday, causing the melt of mountain snow to accelerate. That was expected to push the river up again, with the next peak forecast for Tuesday afternoon, meteorologist Albert Richmond said.

If another surge of water pushes oil further into back channels as expected, it could be a potential threat to fisheries, said Bruce Farling, executive director of Trout Unlimited's Montana chapter. Farling said there are many fish eggs and recently hatched fish in those channels.

The stretch of the Yellowstone where the spill occurred contains sauger, bass catfish, goldeye, trout and, farther downstream, below Miles City, native pallid sturgeon.

"If we get a bunch of oil in some of these backwater areas, these are precisely where these small fish rear," Farling said.