AMHERST, S.D. - TransCanada Corp.'s Keystone pipeline leaked an estimated 210,000 gallons of oil onto agricultural land in northeastern South Dakota, the company and state regulators said Thursday, but state officials don't believe the leak polluted any surface water bodies or drinking water systems.

Crews shut down the pipeline Thursday morning and activated emergency response procedures after a drop in pressure was detected resulting from the leak south of a pump station in Marshall County, TransCanada said in a statement. The cause was being investigated.

Discovery of the leak comes just days before Nebraska regulators are scheduled to announce their decision Monday whether to approve the proposed Keystone XL oil pipeline, an expansion that would boost the amount of oil TransCanada is now shipping through the existing line, which is known simply as Keystone. The expansion has faced fierce opposition from environmental groups, American Indian tribes and some landowners.

Brian Walsh, an environmental scientist manager at the South Dakota Department of Environment and Natural Resources, said the state has sent a staff member to the site of the leak in a rural area near the border with North Dakota about 250 miles west of Minneapolis.

"Ultimately, the cleanup responsibility lies with TransCanada, and they'll have to clean it up in compliance with our state regulations," Walsh said.

TransCanada said in its statement that it expected the pipeline to remain shut down as the company responds to the leak. It did not offer a time estimate.

President Donald Trump issued a federal permit for the expansion project in March even though it had been rejected by the Obama administration. The Keystone XL project would move crude oil from Canada, across Montana and South Dakota to Nebraska, where it would connect with existing pipelines feeding refineries along the Gulf Coast.

Sierra Club Beyond Dirty Fuels director Kelly Martin said in a statement that the only way to protect against leaks is for Nebraska to reject the pipeline.

"We've always said it's not a question of whether a pipeline will spill, but when, and today TransCanada is making our case for us," Martin said.