The US Army Corps of Engineers sent a letter to Northern Dynasty subsidiary the Pebble Limited Partnership on Monday, asking for the first time what the company planned to do to mitigate the disturbance of about 3,000 acres of wetlands and 100 miles of streams.

"A clear reading of the letter shows it is entirely unrelated to recent tweets about Pebble and one-sided news shows. The White House had nothing to do with the letter, nor is it the show-stopper described by several in the news media over the weekend," said PLP CEO Tom Collier.

He said the USACE letter did not ask for more or additional information, but instead for the first time placed on record its formal assessment regarding mitigation for the Pebble project.

"The letter does not ask for a delay or pause in the permitting process. In fact, it clearly states that the USACE is continuing its work toward a record of decision for the project," Collier said.

"This is the next step in what has been a comprehensive, exhaustive two-and-a-half-year review of the project. Nothing in the letter is a surprise to us, or them."

Coinciding with the start of the Republican national convention, the PLP announced it had launched an advertising and outreach campaign to push for the approval of Pebble.

"Based on our understanding of the substance of the letter, our discussions with the state, our substantial work in the field and our discussions with the USACE, we believe our final comprehensive management plan submission will be submitted within weeks and will satisfy all of the requirements of the letter," said Collier.

The company plans to share more details of the mitigation plan soon.

Shares in Northern Dynasty (TSX:NDM) lost more than half their value in early trading Monday to C94c, down from the previous close at $1.91. By the afternoon session, the stock was down 38% at $1.19, tempering year-to-date gains of more than 450% to 112.5%. NDM has a market value of $603 million (US$456 million).