A train carrying potentially hazardous materials derailed into the Yellowstone River in Montana early Saturday, authorities said.

The bridge the train was going over during the incident collapsed, but it's unclear how the incident started, officials said.

Three of the railcars that derailed were carrying hot asphalt, and four were carrying molten sulfur, KC Williams, the director of Emergency Management for Yellowstone County, told ABC News.

Drinking water in Yellowstone County is not currently affected, Williams said.

Montana Rail Link said in a statement Saturday that the incident happened around 6:45 a.m. local time while the train was traveling westbound near the town of Reed Point. Several cars are in the river, including several hazmat cars, the railroad operator said.

The cause of the derailment is under investigation with MRL personnel, and first responders onsite, the operator said. Montana Disaster and Emergency Services and the Environmental Protection Agency's National Response Center have been notified, MLR said.

"The safety of our employees and the public remains our top priority," MRL said. "We are committed to addressing any potential impacts to the area as a result of this incident and working to understand the reasons behind the accident."

The incident near Twin Bridges Road between the towns of Reed Point and Columbus has led to the closure of parts of the Yellowstone and Stillwater Rivers, the Montana Department of Fish, Wildlife and Parks said Saturday.

State officials are advising the public to avoid parts of the Yellowstone River due to "potential contaminants."

There is no immediate threat to Yellowstone County, the sheriff's office said.

ABC News' Samira Said contributed to this report.

This is a developing story. Check back for updates.