WASHINGTON (Reuters) - The Navajo Nation will sue the Trump administration if it tries to reduce the size of the Bears Ears National Monument in Utah, its top lawyer told Reuters on Thursday, ahead of the release of a broad government review of such sites across the country.

FILE PHOTO: Bears Ears, the twin rock formations which form part of Bears Ears National Monument in the Four Corners region, are pictured in Utah, U.S. May 16, 2017. REUTERS/Bob Strong

President Donald Trump had ordered the Interior Department to examine whether 27 national monuments designated by past presidents could be reduced or rescinded to make way for oil and gas drilling and other economic development.

The results have not been announced, but a leak of the review obtained by the Washington Post shows the Interior Department will recommend shrinking some sites, including Bears Ears, a 1.35-million-acre wilderness that the Navajo and other tribes consider sacred.

“We are prepared to challenge immediately whatever official action is taken to modify the monument or restructure any aspect of that, such as the Bears Ears Commission,” Attorney General of the Navajo Nation Ethel Branch told Reuters.

Branch said the tribe believes changing the boundaries of Bears Ears would violate the Antiquities Act, a century-old law that protects sacred sites, cultural artifacts and other historical objects.

Former President Barack Obama designated Bears Ears a national monument under the Antiquities Act shortly before leaving office, at the urging of the Navajo and four other Native American tribes.

The move pleased conservationists, but angered Republican lawmakers from the state. Utah lawmakers want to shrink the monument to one-tenth of its size to make way for expanded economic activities, according to records obtained by the Salt Lake Tribune.

Trump has said past presidents abused the Antiquities Act and put too much land off limits to development.

In a memorandum Interior Secretary Ryan Zinke sent to Trump and leaked to media this week, Zinke recommends shrinking Bears Ears and three other monuments and modifying the way 10 monuments are managed.

In the memo, Zinke recommends Trump ask Congress to pass legislation creating a tribal co-management structure to oversee cultural resources at Bears Ears. Utah Republican Congressman Rob Bishop, chair of the House Natural Resources committee, said he is prepared to introduce such legislation.

Branch said Zinke did not take enough formal input from the governments of the five tribes that form the Bears Ears Commission - the Navajo Nation, the Hopi, Pueblo of Zuni, Ute Mountain and Ute Indian tribes. That commission was “painstakingly designed” by the tribes when the monument was designated, she said.

Heather Swift, spokeswoman for Zinke, said he met with local Navajo members in San Juan County who want to reduce the monument size and that “multiple tribal listening sessions were held throughout the review period.”