AMHERST, S.D. — A federal pipeline safety agency has sent technical experts to the site of an estimated 210,000-gallon oil spill from the Keystone pipeline in northeastern South Dakota.

A Pipeline and Hazardous Materials Safety Administration representative said Friday the agency's investigation is ongoing.

TransCanada Corp. crews shut down its Keystone pipeline Thursday after a drop in pressure was detected from the leak south of a pump station in Marshall County, South Dakota.

State officials say the buried pipeline leak is on agricultural land and don't believe it has polluted any surface water bodies or drinking water systems.

The pipeline delivers oil from Canada to refineries in Illinois and Oklahoma. The leak was found just days before Nebraska regulators are to announce whether they approve an expansion of the Keystone system

TransCanada says it expects the pipeline to remain shut down as the company responds to the leak.

According to U.S. government data, there have been 17 oil leaks in the U.S. larger than the new spill since 2010.

But it comes just four days before Nebraska regulators are due to announce their final decision on whether a major expansion of the pipeline system, called Keystone XL, can pass through the state. The expansion has been fiercely opposed by environmental activists, American Indian tribes and some landowners.

President Donald Trump has approved a permit for the expansion.

More information on the Keystone pipeline inside: page A2.