Mountain snowmelt is making areas damaged by crude difficult to reach.
On Friday, a 12-inch Exxon Mobil pipeline burst upstream from a refinery in Billings.
Exxon Mobil officials estimate up to 1000 barrels, or 42,000 gallons, of oil spilled.
The company promised to do whatever is necessary to mop it up.
After downplaying assertions from state and federal officials that damage from the spill was spread over dozens of miles, Exxon Mobil acknowledged under political pressure that the scope of the leak could be greater.
The Associated Press contributed to this report.