TransCanada has shut down a portion of the Keystone pipeline after finding a leakage of oil in South Dakota.

The company is investigating the source of the oil spill, which is located about four miles away from the Freeman pump station in Hutchinson County, South Dakota.

There is no clear estimate on how much crude oil has been leaked and the company is in the process of removing the oil. The pipeline transports oil from Alberta, Canada to Cushing, Oklahoma.

South Dakota Public Utilities Commission chairman Chris Nelson said: "We’ve been given an early estimate, but until they actually dig down to the pipeline, I don’t think they’re going to have a firm number on the exact number of gallons that were involved."

Calgary-based TransCanada may keep the pipeline closed for a week following the incident. The company was alerted by a landowner over a possible leakage of oil.

"No significant impact to the environment has been observed and our investigation continues," TransCanada officials said in a statement.

The company has notified the state and federal regulators about the incident. It has dispatched ground crew to ascertain the reasons for the leakage.

In 2008, TransCanada announced a proposal to expand the pipeline, which is known as Keystone XL. The pipeline was aimed at transporting the oil directly from Canada to Nebraska.

However, the pipeline expansion was strongly criticized by the environmentalists, even as the company highlighted job creation and national security benefits.

Following a prolonged review, President Obama rejected an approval for the project in late last year. TransCanada has challenged the decision in a federal court.