BISMARCK, N.D. — Owners of North Dakota’s only oil refinery said Friday that gasoline was blended improperly at the facility, prompting complaints from drivers.

Tesoro Petroleum Corp. said in a statement that “the situation was related to off-spec fuel” from its Mandan refinery. The company would not comment on how much fuel was improperly made, how it happened or how many retailers and drivers were affected.

Tesoro said it began receiving complaints about bad gasoline on Tuesday. The company said diesel was not affected.

“We’re still working to determine exactly what happened,” company spokeswoman Jill Saletta said in an email. “We cleared the tank and are working with every customer to remove affected fuel and replace it.”

North Dakota Petroleum Marketers Association President Mike Rud said stations in Bismarck, Mandan and Washburn had reported problems.

Tesoro’s statement said fuel continued to be replaced at affected sites on Friday.

Built in 1954, the Mandan refinery across the Missouri River from Bismarck, produces gasoline, diesel, jet fuel, propane, butane and residual fuel. It also has an ethanol blending facility. Tesoro, which is based in San Antonio, acquired the refinery from BP in 2001.

Most of the Mandan refinery’s products are shipped through pipelines to eastern North Dakota and Minnesota and sold to customers in the Dakotas, Minnesota and Wisconsin, the company has said. Gasoline makes up about 60 percent of the refinery’s production on average.

Bruce Heine, a spokesman for Tulsa, Okla.-based Magellan Midstream Partners, said the pipeline company gets none of its product from Tesoro. Magellan continued to replace gasoline Friday in several Minnesota communities because the fuel was blended with too much ethanol at its Mankato terminal, he said.

“Tesoro gas had nothing to do with it,” he said “They are separate incidents.”