The Trump administration took steps on Monday to significantly weaken the US Endangered Species Act, prompting state attorneys general and conservation groups to threaten legal action to protect at-risk species.

The 1970s-era act is credited with bringing back from the brink of extinction species such as bald eagles, gray whales and grizzly bears, but the law has long been a source of frustration for drilling and mining companies, and other industries because new listings can put vast areas of land off-limits to development.

The weakening of the Act's protections is one of many moves by US President Donald Trump, a Republican, to roll back existing regulations to hasten oil, gas and coal production, as well as grazing, ranching and logging on federal land.

"These changes crash a bulldozer through the Endangered Species Act's lifesaving protections for America's most vulnerable wildlife," Noah Greenwald, the Center for Biological Diversity's endangered species director, said in a statement. "For animals like wolverines and monarch butterflies, this could be the beginning of the end."

The changes would end a practice that automatically conveys the same protections for threatened species as for endangered species, and would strike language that guides officials to ignore economic impacts of how animals should be safeguarded.

The original act protected species regardless of the economic considerations.

"The revisions finalized with this rulemaking fit squarely within the president's mandate of easing the regulatory burden on the American public, without sacrificing our species' protection and recovery goals," US Secretary of Commerce Wilbur Ross said in a statement.

The changes were announced by the Interior Department's US Fish and Wildlife Service (USFWS) and the Commerce Department's National Marine Fisheries Service (NMFS).

Massachusetts and California will lead a multi-state lawsuit joined by conservation groups once the final rule is published in the Federal Register in the coming weeks, challenging what they say was an "illegal" process to revise it.

"By gutting key components of the Endangered Species Act, one of our country's most successful environmental laws, the Trump administration is putting our most imperiled species and our vibrant local tourism and recreation industries at risk," said Massachusetts Attorney General Maura Healey.

"We will be taking the administration to court to defend federal law and protect our rare animals, plants, and the environment," she added on a call with reporters.