The Navajo Nation says it plans to sue the Trump administration if it tries to reduce the size of the Bears Ears National Monument in Utah.

"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," Ethel Branch, the attorney general of the Navajo Nation told Reuters Thursday.

The tribe argues changing the boundaries of Bears Ears would violate the Antiquities Act, a century-old law that permits presidents to unilaterally protect sacred sites, cultural artifacts and other historical objects. Some presidents have reduced the size of monuments, but only on a limited scale. The concept has never been tested in court.

Bears Ears is one of the most sacred centers for American Indian tribes, including the Navajo, Hopi, Uintah and Ouray Ute, Ute Mountain Ute and Zuni. It has more than 100,000 Native American archeological and cultural sites, according to the Bears Ears Coalition.

The Trump administration and some congressional Republicans say recent presidents have abused the Antiquities Act by protecting larger swaths of land than the law allows for.

President Trump ordered the Interior Department to undertake a review of 27 national monuments shortly after his inauguration.

Bears Ears is perhaps the most contentious monument reviewed by Interior Secretary Ryan Zinke, who issued a report to Trump last month with recommendations.

In a memo leaked to the media, Zinke recommends shrinking or changing the boundaries of six national monuments, including Bears Ears, and proposes management changes to four others that could reopen areas to logging, cattle grazing and commercial fishing.

Former President Barack Obama in December, just before he left office, established Bears Ears, a retreat of mesas and canyons in Utah's poorest county, as a national monument.

News reports have said Zinke may call for Trump reduce Bears Ears' 1.35 million acres by up to 88 percent.

The Navajo Nation has led a coalition of tribal leaders who supported the monument designation and are fighting efforts to undo it.

But some local tribes, and most local and national politicians in Utah, opposed Obama's move.