Eastern Montana residents were stocking up on bottled water after authorities detected traces of oil in public water supplies downstream of a Yellowstone River pipeline spill.
Elevated levels of benzene were found in water samples from a treatment plant that serves about 6,000 people in the community of Glendive, near North Dakota.
Scientists from the Centres for Disease Control and Prevention said the benzene levels were above those recommended for long-term consumption, but did not pose a short-term health hazard.
Residents were warned not to drink or cook with water from their taps.
Some criticised the timing of the advisory, which came more than two days after 189,270 litres of oil spilled from the Poplar pipeline owned by Wyoming-based Bridger Pipeline Co. The spill occurred about 8km upstream from the city.
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Adding to the frustrations was uncertainty over how long the water warning would last.
A mechanical inspection of the damaged line on Tuesday revealed the breach occurred directly beneath the river, about 15 metres from the shore, Bridger Pipeline spokesman Bill Salvin said.
The spill's cause remained undetermined.
By Tuesday, oil sheens were reported as far away as Williston, North Dakota, below the Yellowstone's confluence with the Missouri River.
Officials took initial steps on Tuesday to decontaminate the water system. Glendive Mayor Jerry Jimison said it was unknown when the water treatment plant would be back in operation.
Until that happens, Salvin said Bridger Pipeline will provide 37,854 litres of drinking water a day to Glendive.
