JUNEAU — The U.S. Environmental Protection Agency announced it is taking the first steps toward restricting or even prohibiting development of a massive gold-and-copper prospect near the headwaters of a premier sockeye salmon fishery in southwest Alaska — though no final decision has been made.

While the rarely used EPA process is underway, the U.S. Army Corps of Engineers cannot approve a permit for the proposed Pebble Mine project.

The announcement Friday follows release of an EPA report in January that found large-scale mining in the Bristol Bay watershed posed significant risk to salmon and could adversely affect Alaska Natives in the region, whose culture is built around salmon.

EPA Administrator Gina McCarthy made clear Friday that no final decisions have been made. While McCarthy said scientific and other data has provided “ample reason” for EPA to believe a mine of the size and scope of Pebble “would have significant and irreversible negative impacts on the Bristol Bay watershed and its salmon-bearing waters,” she said EPA is open to receiving more information.

Mine opponents have been urging EPA to take steps to protect the region and hailed Friday’s announcement as significant. Supporters of Pebble Mine fear that EPA will move to block the project even before it gets to the permitting phase.

Tom Collier, CEO of the Pebble Limited Partnership, which is working to advance the mine project, called the EPA process a “major overreach.” In a statement, Collier said EPA’s actions to date “have gone well outside of its normal practice, have been biased throughout, and have been unduly influenced by environmental advocacy organizations.”

He said the partnership remained confident in its project — calling it an important asset for the people of Alaska — and said Pebble would continue to make its case with EPA.

McCarthy told reporters on a teleconference that EPA is initiating the process under the Clean Water Act to determine how it can best use its authorities “to protect Bristol Bay rivers, streams and lakes from the damage that will inevitably result from the construction, operation and long-term maintenance of a large-scale copper mine.”

EPA has rarely used this specific authority, which it can exercise before a permit is applied for. The agency says it has only done so 29 times in the past, and in 13 of those cases the EPA decided to take steps to limit or prohibit activity.

In this case, McCarthy said, “the Bristol Bay fishery is an extraordinary resource and is worthy of out-of-the-ordinary agency actions to protect it.”

The watershed produces nearly half the world’s wild sockeye salmon, a fish that is important for Alaska Natives in the region.

McCarthy called the watershed one of most productive ecosystems on the planet.

The Associated Press sought comment from Gov. Sean Parnell’s office.

U.S. Sen. Lisa Murkowski, R-Alaska, said EPA was continuing to move toward a “premature veto based on what it assumes may happen with this project. We already have undeniably grave problems with federal agencies blocking resource production on federal lands in Alaska. Now to see a federal agency overstep its authority and move prematurely to block even the consideration of a permit for potential activity on state lands is something I simply cannot accept.”

Regional administrator Dennis McLerran said information provided by the Pebble Partnership and Northern Dynasty Minerals Ltd., the mine’s owner, showed mining would involve excavating “the largest open pit ever constructed in North America, completely destroying” an area as large as 7 square miles and that disposal of waste material would require building three impoundments covering another 19 square miles.

In a letter to officials with the state, Pebble Partnership and corps, McLerran also said the EPA’s report estimated that discharges of dredged or fill material associated with the footprint of the mine would likely cause “irreversible loss of significant reaches” of salmon- and other fish-supporting streams, as well as extensive areas of wetlands, ponds and lakes. McLerran said the process relates only to the Pebble deposit and not to other mining or non-mining development in the watershed.

Tribes and others petitioned the EPA in 2010 to protect Bristol Bay, a request that gave rise to the watershed assessment released in January.

Friday’s action means the corps, state and those behind the mine project will be allowed to submit information to the EPA to show no “unacceptable adverse effects” to aquatic resources would result from mining-related discharges or that actions could be taken to prevent unacceptable effects to waters, according to the letter. Regulations call for responses within 15 days, though extensions can be granted.

McLerran told reporters this will be an opportunity for the state and Pebble to rebut what EPA has put forth. If McLerran is not satisfied with their response, the agency will publish proposed restrictions or prohibitions on mining at the Pebble deposit and gather public comment.

There will be a second round of consultation before a final decision is made.

The entire review process could take about a year, he said, and the agency, at any point, could decide further action on its part is unnecessary.

The Pebble Partnership has called the mine deposit one of the largest of its kind in the world, with the potential of producing 80.6 billion pounds of copper, 107.4 million ounces of gold and 5.6 billion pounds of molybdenum over decades. Northern Dynasty has been looking for a new partner after Anglo American PLC pulled out last year, citing a prioritization of projects. Pebble has criticized the EPA process as flawed and has said that it has yet to finalize its mine designs.