The Keystone oil pipeline was shut down Tuesday morning after a rupture in North Dakota halted the flow from Canada to U.S. refineries.

South Bow, which manages the pipeline, said it was shut down after a pressure drop was detected by the control center leak detection systems, The Associated Press reported.

“The affected segment has been isolated, and operations and containment resources have been mobilized to site,” the company said. “Our primary focus right now is the safety of onsite personnel and mitigating the risk to the environment.”

The spill was contained to an agricultural field in the area, and no people or structures were impacted, but the cause for the rupture was not immediately known.

Reports said the oil was surfacing 300 yards from the pump station in a field, and emergency services responded.

Bill Suess, a spill investigation program manager with North Dakota’s Department of Environmental Quality, told the AP that an employee heard a “mechanical bang” and shut down the pipeline within minutes.

He noted there was a “fairly good volume” spilled but there have been “much, much bigger spills” in the past and he did not think it was going to be “that huge.”

South Bow has managed the pipeline, which was built in 2010, since last year.

There were plans to expand the pipeline, dubbed Keystone XL. But they were dropped by the Biden administration after it drew criticism from environmentalists. President Trump has recently called for the completion of the controversial pipeline.

The Associated Press contributed.