Pipeline company TransCanada said it closed its Keystone pipeline while it examines what it said may be a small leak from a pump station in South Dakota.

TransCanada said in an emailed statement that a "potential incident" was observed first around midday Saturday about 50 miles west of Sioux Falls, S.D. The company said it began the process of closing down its Keystone oil pipeline after notification of the incident and sent crews to the scene to assess the situation.

"No significant impact to the environment has been observed and there is no threat to public safety or security," the company said in a late Monday statement.

TransCanada said the visible oil from the pumping station covers about 300 square feet. Since Saturday, the company said the size of the impacted area hasn't changed and crews were on location working to excavate soil and examine the underlying pipeline.

The pipeline will remain closed while TransCanada examines the situation. The pipeline runs from Alberta, Canada, to terminals in Cushing, Okla., and Wood River, Ill. That section is directly impacted by the release, though consumers linked to a line stretching from Cushing to the U.S. Gulf Coast remain supplied.

TransCanada in January filed plans to issue a claim under the North American Free Trade Agreement in response to a U.S. federal decision to deny its permit to build an expansion pipeline, Keystone XL. A lawsuit was also filed in a federal court in Houston, arguing U.S. President Barack Obama overstepped his authority in his decision to deny construction of the pipeline.

The company said it continued to support the advancement of Keystone XL. The existing Keystone network in the United States added additional long-term delivery contracts in the fourth quarter.