About 5,000 barrels, or 210,000 gallons of oil, leaked Thursday from a section of the Keystone Pipeline in South Dakota, according to operator TransCanada.
In a statement to its website, Transcanada wrote that crews had shut down the pipeline at 6 a.m. CST after employees identified a drop in pressure.
The company is now working alongside contracted specialists in assessing the situation and providing relevant information to government regulators, according to the statement.
The spill is the largest Keystone leak to occur in South Dakota, spokesman for the state's Department of Environment and Natural Resources Brian Walsh told CNN.
An April 2016 release spilled just 400 barrels, or about 17,000 gallons, of oil. Cleanup from that incident took approximately two months.
The spill comes as American Indian tribes in the Dakotas are renewing efforts to protect natural water supplies against the recently completed Dakota Access Pipeline.
Lawyers for the Standing Rock and Cheyenne River Sioux tribes filed documents Wednesday requesting a federal judge to reconsider the tribes proposals, which federal officials and the pipeline's developer labeled unwarranted.
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