An oil pipeline that runs under the Yellowstone River in the western U.S. state of Montana has ruptured, dumping an unknown amount of oil into the waterway.

The ExxonMobil pipeline ruptured late Friday near the town of Laurel and prompted an evacuation of local residents. Witnesses reported seeing an oil slick in the river.

Exxon officials have not said what caused the leak from the 30-centimeter pipeline. But local officials say the river's high level, the speed of the water and debris may have been factors.

The oil pipeline has been shut down. It originates near the border of Montana and the state of Wyoming and delivers about 60,000 barrels a day to a refinery next to the Yellowstone River.

In a statement, Exxon said it is working hard to mitigate the environmental impacts of the spill.