TransCanada says the damage to the Keystone pipeline has leaked about 16,800 gallons in South Dakota, a dramatic increase from initial estimates of 187 gallons.
The leak was first discovered on Saturday and forced a key section of the pipeline to be shut down.
The company says it has made progress but has “not yet discovered the source” of the leak.
Despite that, TransCanada said the spill has been “controlled” because the pipeline was shut down immediately after it was reported and valves and pump stations were shut down remotely.
“There is no significant environmental impact observed,” TransCanada said.
A crew of about 100 people are working around the clock with federal regulators at the site of the spill, which is located about four miles from its Freeman pump station in Hutchinson County, South Dakota.
TransCanada says about 400 barrels of oil, or 16,800 gallons, were spills.
The company drastically increased its estimates after it safely excavated about 100 feet of pipe.
TransCanada said it has “taken this incident very seriously” and continues to work with federal and state regulatory agencies.
The “disaster is a stark reminder that it’s not a question if a pipeline will malfunction, but rather a question of when,” Michael Brune, executive director of the Sierra Club, wrote in a statement. ”
It was these exact type of environmental concerns that led President Obama to deny an expansion of the Keystone pipeline in 2015.