The Dakota Access Pipeline has suffered its first oil leak before it has become fully operational, with indigenous groups saying it shows how much the project threatens the environment.

Although state officials said the spill on 4 April was relatively small and quickly cleaned up, opponents of the project says it raises further concerns about the hazards to Native American sites.

The chairman of the Standing Rock Sioux tribe, Dave Archambault, said the spills are "going to be nonstop", the pipeline threatens its water and cultural sites, and the courts should intervene.

"With 1,200 miles of pipeline, spills are going to happen. Nobody listened to us. Nobody wants to listen, because they're driven by money and greed," he said.

"Our lawsuit challenging this dangerous project is ongoing, and it's more important than ever for the court to step in and halt additional accidents before they happen – not just for the Standing Rock Sioux Tribe and our resources but for the 17 million people whose drinking water is at risk," he said in a statement.

The pipeline, which will take oil from North Dakota through South Dakota and Iowa to Illinois, got the green light for completion from President Donald Trump.

The project has been beset by protests and a lawsuit by four Sioux tribes wanting to close it. It is set to be fully operational by 1 June.

Jan Hasselman, a lawyer for the Standing Rock Sioux tribe, said: "They keep telling everybody that it is state of the art, that leaks won't happen, that nothing can go wrong. It's always been false. They haven't even turned the thing on and it's shown to be false."

However Energy Transfer Partners said that the pipeline is safe and that spilled oil was contained while no other spills have been reported along the pipeline, the Associated Press reports.

Brian Walsh, an environmental scientist with the South Dakota department of environment and natural resources, said the spill was caused by a mechanical failure at a surge pump, saying: "It's not uncommon to have a small release at a pump station".