TransCanada Corp. has shut down its Keystone crude oil pipeline indefinitely after a leak was detected Saturday afternoon in South Dakota.

The company is investigating the incident near its Freeman pump station, in a remote area of Hutchinson County.

It is not clear how much oil was spilled but cleanup is underway.

"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," said Chris Nelson, chairman of the South Dakota Public Utilities Commission.

TransCanada is in the process of removing the oil and investigating the source of the leak, reported at 4 p.m. Saturday.

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

The pipeline remains shut down from Hardisty, Alta., to Wood River, Ill., and from Steele City, Neb., to Cushing, Okla. However, the Gulf Coast line, from Cushing to Nederland, Texas, remains operational.