Environmental activist group Greenpeace said the leak demonstrated that approval should not be given for another section of the 2,600-mile pipeline planned for Nebraska.

At least 210,000 gallons of oil have leaked from the Keystone Pipeline in South Dakota in the United States, just days before a crucial decision due on whether to grant a permit for a long-delayed sister pipeline.

The spill, which amounts to some 5,000 barrels, is the largest oil spill to date in South Dakota, said a spokesman for the state’s Department of Environment and Natural Resources.

Crews immediately shut down the Marshall County pipeline on Thursday morning, and officials are now investigating the cause of the leak. So far, there have been no reports of damage to waterways or wildlife, CNN reports.   

The pipeline’s operator, TransCanada, said in a statement: "The safety of the public and environment are our top priorities and we will continue to provide updates as they become available."

But environmental activist group Greenpeace said the leak demonstrated that approval should not be given for another section of the 2,600-mile pipeline, which delivers crude oil from Canada to Texas, to be built in Nebraska.

In March, President Donald Trump's administration officially issued a permit that approved construction of the Keystone XL Pipeline.

Environmentalists oppose the project because it would cut across the Ogallala Aquifer, one of the world's largest underground freshwater deposits. And Native American groups have argued the pipeline would cut across their sovereign lands.

"The Nebraska Public Service Commission needs to take a close look at this spill," said Greenpeace spokesman Rachel Rye Butler. "A permit approval allowing Canadian oil company TransCanada to build Keystone XL is a thumbs-up to likely spills in the future."

Jane Kleeb, head of the Nebraska Democratic Party and a longtime activist opposed to Keystone XL, told the Washington Post: "TransCanada cannot be trusted. I have full confidence that the Nebraska Public Service Commission is going to side with Nebraskans, not a foreign oil company."

TransCanada said the latest leak occurred about 35 miles south of the Ludden pump station, which is in southeast North Dakota, and that it  was "completely isolated" within 15 minutes.

The company said it has obtained permission from the landowner to assess the spill and plan the clean-up operation.