A pipeline operated by Exxon Mobil Corp in Montana state ruptured late Friday, dumping as many as 1,000 barrels of crude oil into the Yellowstone River over the weekend.

The company was able to shut down the valve within six minutes of computers sensing a pressure change. The cause of the leak remains unclear.

Exxon estimated the oil release at anywhere from 750 to 1,000 barrels ((32,000 to 42,000 gallons) and the pipeline typically transports 40,000 barrels a day.

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. But Montana Governor Brian Schweitzer said the oil's spread in the Yellowstone -- which is the longest undammed river in the United States - could be more extensive, Reuters reported.

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. A full assessment of the oil's spread will not be possible until small boats can be deployed in the river, he said.

The company said more than 100 people were working on the oil spill by the end of Sunday. It has deployed oil-absorbent pads and boom to aid in cleanup efforts.

Meanwhile the county's emergency services director Duane Winslow said flooding has made it harder to track and clean up the mess. The Yellowstone was running above flood stage over the weekend, sweeping brush and logs into the river, and had a 5- to 7-mph current Sunday, CNN reported.

It's too dangerous to do anything on the river, to put out any sort of boats or anything. So people will be working from the shores rather than out in the middle of the river,” he said.

The accident happened from a 12” crude pipeline operated by ExxonMobil Pipeline Company that runs from Silver Tip, Montana to Billings, Montana. All appropriate state and federal authorities have been alerted, the company said in a statement.

At this early stage, we have no information on the cause of the incident, and we are working to determine the amount of oil released. ExxonMobil Pipeline Company deeply regrets this release and is working hard with local emergency authorities to mitigate the impacts of this release on the surrounding communities and to the environment, the company said.

Exxon promised a full investigation into the spill, which occurred in a 12-inch pipeline, running downstream from U.S.'s major tourist attraction Yellowstone national park.

We recognize the seriousness of this incident and are working hard to address it. Our principal focus is on protecting the safety and health of the public and our employees. We will, of course, also begin a thorough investigation of the cause of this unfortunate event. We will provide additional information as it becomes available, the company said.