Crews worked on Monday to clean up crude oil that spilled in and near the Yellowstone River in eastern Montana while officials with Bridger Pipeline LLC tried to determine what caused the weekend breach.

Bridger has said the break in the 12in steel pipe happened on Saturday morning in an area about 9 miles upstream from Glendive. Bridger’s spokesman, Bill Salvin, said on Monday that the company is confident that no more than 1,200 barrels – or 50,000 gallons – of oil spilled during the hour-long breach.

“Oil has made it into the river,” Salvin said. “We do not know how much at this point.”

Oil has been seen in the river in spots 15 and 25 miles downstream from Glendive, Salvin said. Some of the oil is trapped under ice.

The company, which is based in Casper, Wyoming, is testing the water in Glendive for any contamination.

The Poplar Pipeline system runs from Canada to Baker, Montana, and carries crude oil from the Bakken oil producing region in Montana and North Dakota. It remained shut down on Monday. It receives oil at the Poplar Station in Roosevelt County, Fisher and Richey Stations in Richland County and at Glendive in Dawson County, all in Montana.

The pipeline was last inspected in 2012, Salvin said, and is at least 8ft below the Yellowstone river bed where it crosses the river near Glendive.

Bridger Pipeline, a subsidiary of True companies, also owns and operates the Four Bears pipeline system in North Dakota along with the Parshall gathering system and the Powder river system in Wyoming, according to the company’s website.

Montana officials said on Sunday that they were unaware of any threats to public safety or health. Some of the oil did get into the water, but the area where it spilled was frozen over and that could help reduce the impact, said Dave Parker, a spokesman for Governor Steve Bullock.

“We think it was caught pretty quick, and it was shut down,” Parker said. “The governor is committed to making sure the river is cleaned up.”

“Our primary concern is to minimise the environmental impact of the release and keep our responders safe as we clean up from this unfortunate incident,” said Tad True, vice-president of Bridger.

The EPA and state department of environmental quality have responded to the area about nine miles upriver from Glendive, Parker said.

An Exxon Mobil pipeline broke near Laurel during flooding in July 2011, releasing 63,000 gallons of oil that washed up along an 85-mile stretch of riverbank.

Montana officials are trying to determine if oil could have been trapped by sediment and debris and settled into the riverbed.

Exxon Mobil is facing state and federal fines of up to $3.4m from the spill. The company has said it spent $135m on the cleanup and other work.

Montana and federal officials notified Exxon that they intend to seek damages for injuries to birds, fish and other natural resources from the 2011 spill. The company also is being asked to pay for long-term environmental studies and for lost opportunities for fishing and recreation during and since the cleanup.