Nearly 1 million litres of oil have leaked from the controversial Keystone Pipeline in South Dakota.
CNN reports around 800,000 litres leaked before crews were able to shut it off - enough to fill an Olympic swimming pool about a third of the way up.
The cause of the leak is yet to be determined.
"The safety of the public and environment are our top priorities and we will continue to provide updates as they become available," a spokesperson for pipeline owner TransCanada said.
Environmentalists said it was more proof an extension to the pipeline - Keystone XL - should not go ahead, calling it a "thumbs-up to likely spills in the future".
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The existing pipeline runs east from Alberta, Canada, through the provinces of Saskatchewan and Manitoba, then turns south and passes through the US states of North and South Dakota, Nebraska, Kansas, Oklahoma and Texas.
The Keystone XL pipeline would run a more direct route between Alberta and Kansas. It was stopped by former US President Barack Obama, but resurrected by Donald Trump. It was vociferously opposed by environmental groups and Native American tribes.
Mr Trump said in a Twitter post in 2015 it would provide "thousands of jobs" and was "good for the environment, no downside".
The latest spill pales in comparison to the Deepwater Horizon spill, in which 800 million litres of oil was released - 1000 times more.