The Dakota Access pipeline has suffered its first leak, outraging indigenous groups who have long warned that the project poses a threat to the environment.

The $3.8bn oil pipeline, which sparked international protests last year and is not yet fully operational, spilled 84 gallons of crude oil at a South Dakota pump station, according to government regulators.

Although state officials said the 6 April leak was contained and quickly cleaned, critics of the project said the spill, which occurred as the pipeline is in the final stages of preparing to transport oil, raises fresh concerns about the potential hazards to waterways and Native American sites.

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

The pipeline, scheduled to transport oil from North Dakota to Illinois, inspired massive demonstrations in 2016 and was dealt a major blow when the Obama administration denied a key permit for the project toward the end of his presidency. But shortly after Donald Trump’s inauguration, the new administration ordered the revival of the pipeline and worked to expedite the final stage of construction.

The Standing Rock tribe, which has fought the pipeline corporation Energy Transfer Partners and the US government in court, has argued that the project requires a full environmental study to assess the risks of the pipeline. But under Trump, who has close financial ties to the oil company, the project recently completed construction by the Standing Rock tribe’s reservation in North Dakota and has been loading oil in preparation for a full launch.

The April spill, which was first uncovered this week by a local South Dakota reporter, illustrates the need for the more robust environmental assessment that the tribe has long demanded, said Hasselman.

“It doesn’t give us any pleasure to say, ‘I told you so.’ But we have said from the beginning that it’s not a matter of if, but when,” the Earthjustice attorney told the Guardian on Wednesday. “Pipelines leak and they spill. It’s just what happens.”

Brian Walsh, an environmental scientist with the South Dakota department of environment and natural resources, said the spill was relatively minor and was caused by a mechanical failure at a surge pump.

“It’s not uncommon to have a small release at a pump station,” he said, adding that the company responded immediately and cleaned up the liquid petroleum. The spill occurred inside a “secondary containment area” and there were no environmental impacts, he added.

Standing Rock Sioux tribe chairman Dave Archambault II said the spill is another sign that the courts should intervene.

“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 company and the state made no announcements about the spill after it occurred.

Walsh said the department only releases public notices of spills when there is an imminent threat to a waterway or public health. This was the pipeline’s first spill in the state, he said.

Energy Transfer Partners did not immediately respond to the Guardian’s request for comment. A spokeswoman told the Associated Press that the corporation maintains that the pipeline is safe and that the leak was contained in the proper manner. The Associated Press also reported that no other Dakota Access spills have been documented in any other states.

The company has fought in court to keep information about the status of the project confidential.

Hasselman said these kinds of spills should be immediately disclosed.

“What kind of oversight and accountability is there if no one even finds out about these things until weeks later?”