Exxon Mobile, the U.S. Environmental Protection Agency and state authorities are responding to an oil spill from the 12-inch Silvertip pipeline into Yellowstone River.
Exxon said the pipeline leaked for about 30 minutes, though the National Wildlife Federation said federal documents indicate it took about an hour to shut the pipeline.
Flooding along the river had hampered cleanup and recovery operations and Exxon said it was investigating the cause of the spill.
Pipeline leaks in Michigan last year brought renewed criticism of the regional energy infrastructure.
Canadian pipeline company TransCanada wants to build a 1,700-mile extension to the Keystone pipeline that would carry oil sands from the Canadian border in Montana to refineries on the coast of the Gulf of Mexico.
A U.S. House subcommittee on energy passed a measure in June that calls on the government to make a decision on the pipeline by November.
NWF Vice President Jeremy Symons said legislation on the Keystone pipeline should be put on hold until investigators are done examining the Silvertip leak.
"Clearly, the legislation to rush the Keystone XL tar sands pipeline should be postponed until we know the cause of the Yellowstone River oil spill and deal with pipeline safety issues," he said in a statement. "We need a full investigation into the safety of this and other pipelines, updated pipeline safety regulations and stronger oversight."