HOUSTON -- Montana authorities estimate that about 1,000 barrels of crude have leaked out of an Exxon Mobil Corp. pipeline into the Yellowstone River, a state official said Saturday. The pipeline has been shut down.

Tim Thennis, a spokesman for the Montana Disaster and Emergency Services division, provided that initial estimate. He added that although no cause of the spill has been determined, it's possible that heavy flooding affecting that part of the U.S. could have played a part. Thennis said that flooding is also interfering with the clean-up effort, meaning the oil could reach the Missouri River, of which the Yellowstone is a tributary, making the task even more difficult for emergency responders. Montana emergency officials have notified officials in North Dakota that the oil could be heading their way, Tennis said.

"There's no way to capture [the oil] right now," Thennis told Dow Jones Newswires. "The further it spreads the more difficult it becomes."

The Texas-based oil giant said it discovered the leak early Saturday morning. Exxon said it "deeply regrets this release" and that it is working with local authorities to mitigate the impact. The leak took place in a 12-inch crude pipeline that runs from Silver Tip, Mont., to Billings.

The incident comes amid heightened concerns about pipeline safety stemming from natural gas pipeline explosions across the U.S., and from a major spill in July 2010, in which 20,000 barrels of oil escaped from an Enbridge Energy Partners LP pipeline in Michigan.