The US interior secretary recommends reducing the size of at least four of 27 national monuments, reports say.
Ryan Zinke wants to shrink Utah's Bears Ears and Grand Staircase Escalante, Oregon's Cascade-Siskiyou and Nevada's Gold Butte, a leaked report states.
Mr Zinke also proposes lifting restrictions on activities such as logging and mining at 10 sites.
The White House refused to comment on leaked documents - "especially internal drafts which are still under review".
In April, President Trump ordered a review of national monuments - which protect lands for their natural beauty and historical significance - as part of his campaign promise to develop federal land.
He accused previous presidents of improper "land grabs" in establishing the monuments, singling out President Obama who placed 553 million acres under protection during his presidency - more than any other US president.
Environmentalists say reducing restriction, and the size of protected areas, would harm the ecosystems the monuments are intended to protect.
The Wilderness Society said the reported recommendations were "an unprecedented assault on our parks and public lands".
The leaked report was submitted to the White House last month.
Mr Zinke told the Associated Press news agency that none of the sites under review would be transferred to new ownership, but increased public access for activities such as fishing and hunting would be a priority.
The Department of Interior had previously announced no changes would be made to six national monuments in Mr Zinke's home state of Montana, Colorado, Idaho, California, Arizona and Washington.
In the leaked copy of the recommendations Mr Zinke reportedly calls for the easing of regulations on grazing, logging, coal mining and commercial fishing.
He suggests reducing the size of two marine monuments in the Pacific Ocean, the Pacific Remote Islands and Rose Atoll, as well as changing how some monuments are managed.
The report also calls for altering Maine's Katahdin Woods and Waters, New Mexico's Organ Mountains-Desert Peaks and Rio Grande del Norte as well as another marine monument off the New England coast.
Grand Staircase Escalante and Bears Ears, which sits on sacred ground, stretch across a total of more than 3.2 million acres of land while Katahdin spans at least 87,500 acres.
According to Mr Zinke's report, the Grand Staircase Escalante contains "an estimated several billion tons of coal and large oil deposits".
Mr Zinke also suggests exploring the possibility of recognising three new national monuments: Kentucky's Camp Nelson, an 1863 Union Army outpost where African American regiments trained; the Mississippi home of murdered civil rights leader Medgar Evers; and the Badger-Two Medicine area in Montana, which is sacred ground for the Blackfeet Nation.