“It sounds like they’ve got the creek pretty well protected from migration going downstream,” Suess said.

Winter weather was affecting the company’s ability to estimate how much oil had been released, said Wendy Owen, spokeswoman for True Companies.

“We do have a crew on the ground that is doing their best to assess the entire situation and what the next step will be,” she said.

The incident was reported by a landowner who saw oil leaking from the 6-inch pipeline into the creek, Suess said.

A labor union, which opposed a True Companies pipeline under consideration by the North Dakota Public Service Commission in 2015, issued a statement Tuesday urging regulators to have stronger oversight over the company.

The Laborers District Council of Minnesota and North Dakota, which has members working on the Dakota Access Pipeline, said True Companies has a poor track record of spills and environmental incidents. Dakota Access protesters camped in south central North Dakota often cite the company’s 30,000-gallon oil spill in the Yellowstone River as an example of why they oppose a pipeline crossing the Missouri River north of the Standing Rock Sioux Reservation.