Rows of pipe ready to become part of the Keystone Pipeline are stacked in a field near Cushing, Oklahoma.

"Crews, including TransCanada specialists from emergency management, engineering, environmental management and safety as well as contracted, nationally recognized experts are assessing the situation."

According to TransCanada, the pipeline was shut down at 6 a.m. following a recorded drop in pressure. The leak, it said, "was completely isolated within 15 minutes and emergency response procedures were activated."

TransCanada, the company that operates the pipeline, said in a statement that about 5,000 barrels of oil had leaked in Marshall County, South Dakota. One barrel holds 42 gallons, meaning the spill involved roughly 210,000 gallons of oil.

More than 200,000 gallons of oil spilled Thursday from the Keystone pipeline in South Dakota, the operator said, prompting a shutdown of the system.

Image of Amherst incident taken earlier today by aerial patrol as part of our initial response. For more updates, v… https://t.co/u4Vt8aLLR6

The leak happened in a rural area about 3 miles southeast of the town of Amherst. Images from the scene showed a group of trucks congregated along a rural road.

"We’re not quite sure why there was a time gap in there" between when TransCanada shut off the pipe and when it notified the government agency, Walsh said.

Brian Walsh, an official with the South Dakota Department of Environment and Natural Resources, told BuzzFeed News that TransCanada notified his department about the leak at 10:30 a.m. Thursday.

More TransCanada crews showing up to help clean up the pipeline leak from this morning. #ksfynews

David Flute, chairman of the Sisseton-Wahpeton Oyate tribe, told BuzzFeed News the leak was on a section of pipeline adjacent to his reservation. He said the area has "the cleanest lakes in South Dakota," as well as a large subterranean aquifer, and is concerned about the possibility of contamination.

"I’m thinking there is going to be an impact, some type of environmental impact," Flute said. "As the oil seeps, if they can’t contain the spill, which I’m hoping they do — if they’re unable to contain it from seeping into the water systems, it can be hurtful and harmful to everybody."

Walsh said the spill had not affected any bodies of water on the surface, meaning oil "won’t be traveling along a river or a creek." He said the oil was unlikely to reach the aquifer in the area, which is located between 800 and 900 feet underground.

It is unclear how long it will take to clean up the spill. TransCanada did not respond to multiple BuzzFeed News requests for comment.

The pipeline where the spill took place is part of the Keystone Pipeline System, which according to TransCanada stretches across 2,687 miles. An extension of the system, called the Keystone XL, was the source of controversy and protests for years before the Obama administration finally rejected it in 2015.

Earlier this year, however, the Trump administration approved the project.

A number of environmental groups condemned TransCanada and the pipeline in the wake of Thursday's spill, including Friends of the Earth, NextGen America, and the Center for Biological Diversity.

"This disastrous spill from the first Keystone pipeline makes clear why Keystone XL should never be built,” Jared Margolis, an attorney at the Center for Biological Diversity, said in a statement. "Trump’s issuance of a permit for Keystone XL is a farce that will only lead to more pollution for people and wildlife."

The spill also comes just four days before regulators in Nebraska are set to decide if they will allow the Keystone XL pipeline to pass through their state.

Walsh said the section where Thursday's leak occurred began operations sometime around 2010, and Flute said it was greeted with less controversy than the XL project.

The pipeline has leaked before. In April 2016, TransCanada told regulators 187 gallons of oil had spilled, but later revised that number up to nearly 17,000 gallons.