The Trump administration delayed a key permit for the proposed Pebble Mine in Alaska on Monday, saying the company that wants to build the biggest gold and copper mine in North America needs to take extensive action to offset the harm it will cause to the environment.

The decision by the U.S. Army Corps of Engineers marks another reversal for the project, which had been blocked by the Obama administration, then revived by the Trump administration, only to be opposed again recently by members of President Trump’s inner circle, including Donald Trump Jr., who enjoys fishing in the area that would be affected by the mine.

In a statement Monday, the Corps said the project as currently designed would not be allowed under federal law. While the Trump administration “supports the mining industry,” the Pebble Mine proposal would be too damaging to the Bristol Bay region in southwest Alaska, the Corps said.

“Therefore, the Corps finds that the project, as currently proposed, cannot be permitted under section 404 of the Clean Water Act,” it said.

Pebble Limited Partnership must outline how it will address the damage it will do to nearby wetlands and waterways as it extracts gold, copper and other minerals from what would be North America’s largest mine, according to Monday’s letter from the Corps. The request means that a final “record of decision” will be delayed for at least several weeks.

After examining the project’s environmental impact for two years, the Corps concluded late last month that the operation would have “no measurable effect” on the area’s fish populations, but would inflict permanent damage on the region. The company must show within 90 days how it will compensate for harming more than 2,800 acres of wetlands, 130 miles of streams and more than 130 acres of open water within Alaska’s Koktuli River Watershed, the agency said Monday. The mine site is located at the river’s headwaters.

White House officials privately expressed skepticism that the firm could meet the Corps’ requirements and receive a final permit, according to two individuals familiar with the discussion who spoke on the condition of anonymity because they were not authorized to discuss internal deliberations.

The recent push by some of the president’s confidants reinforced existing opposition to the project among staff at the White House National Economic Council, according to one of these individuals. These economic officials, who have raised concerns about it for more than a year, viewed allowing the company to apply for a permit as the right procedural move but not as the final decision.

The company is seeking permission for a 20-year operation that would span more than 13 miles and require the construction of a 270-megawatt power plant, natural gas pipeline, 82-mile double-lane road, elaborate storage facilities and the dredging of a port at Iliamna Bay.

In a statement Monday, Pebble CEO Tom Collier confirmed his firm had received the letter but said it does not mean the project has suffered a setback.

“The letter does not ask for a delay or pause in the permitting process,” he said. “This is the next step in what has been a comprehensive, exhaustive 2½-year review of the project. Nothing in the letter is a surprise to us or them.”

Collier added the company’s plan for offsetting its impact “will be submitted within weeks and will satisfy all of the requirements of the letter.”

The Obama administration had blocked the mine several years ago, on the grounds that it posed a risk to the world’s largest sockeye salmon run. Trump officials reversed that decision, however, allowing the firm to apply for a federal permit.

In recent weeks several well-connected Republicans, including the president’s eldest son, Fox News host Tucker Carlson and Vice President Pence’s former chief of staff Nick Ayers, have campaigned against the project.

Chris Wood, president of the conservation group Trout Unlimited, said it will be difficult for the company to demonstrate how it would make the region’s environment whole, given that aside from a few fishing camps, “the area is pristine. It’s hard to mitigate pristine.”

“The fat lady may not be singing yet, but she’s humming a few bars,” Wood said, adding that the Corps had laid out a scientifically rigorous target. “This is an example of the process working.”

Senior White House aides traveled to Alaska last year and examined the potential project. Andrew Olmem, who served until this year as deputy director of the NEC, and Francis Brooke, another top White House economic official, raised concerns about the mine’s potential environmental impact, said one individual familiar with the matter.

Tribes in the region hailed the Corps’ move Monday.

“We are thankful the Corps has come to the same conclusion as the rest of the scientific community, that Pebble will have such severe impacts there is no way to mitigate the destruction it will cause,” said Alannah Hurley, executive director of the United Tribes of Bristol Bay, in an email. “It is impossible for the company to mitigate the devastation this mine will have on our Native cultures, our way of life that has been sustained for thousands of years by the pristine lands and waters of the Bristol Bay watershed.”