"Zinke is treating our national monuments like contestants on a reality TV show, and his anti-public lands allies in Congress are enabling this unsafe agenda", said Gene Karpinski, president of environmentalist group The League of Conservation Voters, which spent $100,000 on a pro-monument campaign last week.
Zinke told The Associated Press that unspecified boundary adjustments for some monuments designated over the past four decades will be included in the recommendations he planned to give President Donald Trump on Thursday.
Zinke, a former Montana congressman, insisted that public access for uses such as hunting, fishing or grazing would be maintained or restored. "I do not believe the administration can legally shrink any monument designation, and I intend to pursue all available avenues to fight what would be an unprecedented attack on our public lands".
Today's news was a blow to environmental groups that have long expressed fears that the lands could be sold to private development.
When the White House first announced the review, environmentalists sounded the alarm, concerned that it was the beginning of opening up previously-protected territories to oil and gas industry exploitation.
Even though Zinke has stated on the record that many tribes are happy with the new measures taking place, both Sioux and Navajo nation leaders say they have never been happy with the secretary.
Rep. Rob Bishop (R-UT), chairman of the House Natural Resources Committee, reportedly said on a conference call that the White House would need "time to digest" the review before making it public.
Some 2.4 million public comments, almost all of them supporting the national monuments - including the dazzling and historically important geological formations of Grand Staircase-Escalante and Bears Ears in Utah - were submitted in the past 60 days.
The Massachusetts Lobstermen's Association and the Rhode Island Fishermen's Alliance, two of several fishermen's groups suing to undo the creation of the Northeast Canyons and Seamounts Marine National Monument, said they hope the federal government will remove protections on areas where they harvest fish.
Significant reductions in the size of the monuments, especially those created by Obama, would mark the latest in a string of actions where Trump has sought to erode his Democratic predecessor's legacy. "That narrative is patently false and shameful".
Through the years, U.S. presidents of both parties have used the Antiquities Act of 1906 to set aside swaths of public land to protect important historical, cultural, and ecological sites without approval from Congress.
It went on to state, "The responsibility of protecting America's public lands and unique antiquities should not be taken lightly; nor should the authority and the power granted to a President under the Act". Zinke didn't specify which, but based on an interim report released in June, he may recommend that Bears Ears National Monument in Utah be shrunk in size.
Any changes are certain to be challenged in the courts, said Matt Lee-Ashely of the left-leaning Center for American Progress, who noted that any politician who moves to change boundaries is likely to find "a political firestorm on your hands" from nearby communities. He described the concept of the review was "breath of fresh air" that allowed ranchers to contribute to designation of public lands.
The Trump administration has been gearing up for a major review of the country's national monuments for some time now, and the expectation was that plenty of them would be eliminated and opened up to mining, drilling, and deforestation.
Zinke declined to reveal his recommendations for individual sites, however. "Secretary Zinke claims to support public lands, but now we know he's just one more Trump Administration stooge for polluting special interests", Sierra Club executive director Michael Brune said.