Pipeline company TransCanada said clean up operations will continue in South Dakota for several days as it restarts the Keystone oil pipeline after a release.

Survey crews working for the company confirmed a leak on the Keystone pipeline at a pump station in southeast South Dakota. About 400 barrels of oil leaked into the surrounding area.

The company closed the pipeline for about a week after discovering the release in early April. According to the terms of an order from the federal Pipeline and Hazardous Materials Administration, the company restarted the pipeline during the weekend at a reduced rate.

"Clean up and land restoration has already started and will continue during and after our on-site investigation and repair is complete," the company said in an emailed statement.

Nevertheless, TransCanada maintained there was no significant environmental impact observed and no threat to public safety from the release.

The PHMSA, meanwhile, said it was investigating the cause of the leak. Formal penalties or charges may be filed if the company is determined to have violated any pipeline standards, the agency said.

The Keystone pipeline runs from Alberta, Canada, to terminals in Cushing, Okla., and Wood River, Ill.

TransCanada had a permit to build Keystone XL, an extension to the original pipeline, denied by the U.S. government in part on environmental grounds. Early critics of the project pointed to a history of breaks along the current route and raised concerns about the potential harmful environmental impact of heavy crude oil from Alberta.