The Dakota Access Pipeline leaked 84 gallons of crude oil at a pump station in South Dakota last month, according to state documents, just weeks before the pipeline is set to start commercial service.
The spill, the equivalent of two barrels of oil, occurred on April 4 in Tulare township in Spink County, according to South Dakota's Department of Environmental and Natural Resources.
The $3.8 billion project drew environmental protesters from around the world after the Standing Rock Sioux tribe said the pipeline would desecrate a sacred burial ground and that any oil leak would poison the tribe's water supply.
Energy Transfer Partners, the company behind the construction of the pipeline, received approval from the U.S. Army Corps of Engineers in early February after months of delays. It is currently line filling and will be in service on June 1.
After the April 4 spill was reported, recovered oil was put back into the system. Any gravel or soil that had oil was cleaned and disposed of, said Brian Walsh, an environmental scientist with the agency.
The leak occurred some 100 miles (160 km) east of Lake Oahe, a part of the Missouri River system that has been the focal point of the protests.
"This is what we have said all along: oil pipelines leak and spill," Standing Rock Sioux Chairman Dave Archambault II said in a statement on Wednesday.
The Tribe is involved in a lawsuit challenging the project.
The 1,172-mile (1,885-km) Dakota Access line runs from western North Dakota to Patoka, Illinois, where it will link up with another pipeline to bring shale oil from North Dakota's Bakken play to the Gulf Coast.
An Energy Transfer Partners spokeswoman, Vicki Granado, said the spill occurred during the pipeline's commissioning activities. She added the spill occurred in a containment area, so there was no impact on the wider area.