Officials said Monday that they were bringing truckloads of drinking water to the eastern Montana city of Glendive after traces of about 50,000 gallons of oil that spilled into the Yellowstone River were found in the city's water supply.

Preliminary tests at the city's water treatment plant indicated that at least some oil got into a water supply intake along the river, according to state and federal officials. About 6,000 people are served by the intake, Glendive Mayor Jerry Jimison said.

Officials stressed that they were bringing in the shipments of drinking water as a precaution and did not know yet whether there was any health threat. Results of further tests to determine the scope of the danger were expected in coming days.

Up to 50,000 gallons of oil spilled in the Saturday pipeline accident. Cleanup crews were being hampered by ice that covers most of the river, making it hard to find the oil.

Officials with Bridger Pipeline of Casper, Wyoming, said the break in the 12-inch steel pipe happened Saturday morning in an area about 9 miles upstream from Glendive, a community in east-central Montana near the border with North Dakota.

Montana Gov. Steve Bullock toured the spill site Monday afternoon. He said he expected Bridger to continue its cleanup efforts "until it's cleaned up to our standards."

Earlier in the day, Bridger spokesman Bill Salvin said that the company is confident that no more than 1,200 barrels — about 50,000 gallons — of oil spilled during the hourlong breach.

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

An oil sheen was seen near Sidney, almost 60 river miles downstream from Glendive, said Paul Peronard, the on-scene coordinator for the U.S. Environmental Protection Agency.

Booms are being placed in two areas of open water to try and trap oil with another collection site near Crane, which is about 30 miles downstream from the spill site.

"We want to put up a backstop so no free oil can get past this location," Peronard said Monday.

But locating the rest of the oil could prove to be difficult because some of it is trapped under the ice that covers much of the river.

"We really can't see it, so we're going to have to hunt and peck through ice to get it out," Peronard said.

Bridger Pipeline crews were still working Monday to determine exactly where the breach occurred.

If it happened on the bank, some of the oil may be trapped in the soil near the river.

"If it happened underneath the river, then it's all in the river," Peronard said.

Initial water samples taken at the Glendive water treatment facility showed no sign of oil or gas contamination, said Peronard and Dave Parker, Bullock's spokesman.

Glendive's intake station draws water from 14 feet beneath the river surface, while most of the oil was expected to be floating, Peronard said.

Some Glendive-area residents had reported an odor in their water and those reports are being investigated, officials said.

Glendive City Councilman Gerald Reichert said he first noticed an odor in the water at his house Sunday night. He said it smelled like diesel fuel.

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 Monday while crews planned to pump out any remaining oil from the section of the pipeline where the breach occurred.

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

Bridger Pipeline, a subsidiary of True Cos., 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.

The Associated Press