US environmental authorities made a rare move Friday to block a massive copper and gold mine in Alaska before it even gets under way, in a bid to protect wild salmon.

The Pebble Mine project has the potential to be one of the biggest open pit copper mines, but once built, it could threaten the exceptionally rich salmon fishery in the Bristol Bay area, the Environmental Protection Agency explained in a statement.

"Extensive scientific study has given us ample reason to believe that the Pebble Mine would likely have significant and irreversible negative impacts on the Bristol Bay watershed and its abundant salmon fisheries," said EPA Administrator Gina McCarthy.

The EPA action could lead to an unprecedented federal ban on a mining project before the company behind it even puts in a permit request.

"This process is not something the agency does very often, but Bristol Bay is an extraordinary and unique resource," McCarthy said, calling it the "world's most productive salmon fishery."

Bristol Bay produces half the wild sockeye salmon in the world, an average of 37.5 million fish per year, in part because of the exceptional water quality in the streams and wetlands there.

Plans call for the mine to be built near the headwaters of two rivers whose fisheries produce about $480 million of fish and employ some 14,000 people.

In order to move forward, the mine's promoters would need to show that their activities wouldn't threaten the ecosystem.

The EPA move is a victory for environmental activists, fisheries and indigenous groups who had been fighting the mining project for the past three years.

But Republican leaders in Alaska, including Governor Sean Parnell and Senator Lisa Murkowski disapproved, with Murkowski warning the EPA action could set a "terrible precedent."

Promoters of the Pebble project say the region holds one of the world's largest and richest mineral gold and copper deposits, which could produce 36 million tonnes of copper and more than 3,000 tonnes of gold within the next 30 years.

The EPA carried out a large-scale study starting three years ago to evaluate the potential impact of mining on the Bristol Bay ecosystem.

The final report was completed in January and contains 1.1 million public comments as well as the analysis of two independent expert panels.