Mexico has urged the US to alter its plans for expanded fences along their shared border, saying they would damage the environment and harm wildlife.
The affected region includes desert, mountains and wetlands
The fences threaten unique ecosystems, Mexican environment officials warned.
Mexico was ready to file a complaint with the International Court of Justice over the matter if the US did not respond, the environment minister said.
The planned barriers aim to curb illegal immigration, a highly divisive and controversial issue in the US.
The fences, planned along a possible 700 miles (1,125km) of the border, are to be equipped with hi-tech surveillance equipment, including sensors and strong lights.
"The eventual construction of this barrier would place at risk the various ecosystems that we share," Mexico's Environment Minister Juan Rafael Elvira told a news conference.
Those areas include Baja California, Sonora and Arizona, home to one of the world's most important desert ecosystems - the Sonora Desert.
Officials said Mexico was prepared to file a complaint with the International Court of Justice but wanted to explore alternatives first.
Most of the illegal immigrants enter the US via the Mexico border
A report prepared for the Mexican government by experts and environmental activists from Mexico and the US said the barriers could isolate border animals into smaller groups, affecting their genetic diversity.
These include jaguars, Mexican black bears and the endangered antelope-like Sonora Pronghorn.
The use of intense lights and radar could also affect nocturnal species, they said.
The report suggested ways of minimising environmental damage, including "green corridors" of wilderness without roads.
These would allow wildlife to remain connected but not provide an easy route for people trying to cross.
Another proposal was "live" fences of cactuses, or permeable barriers to allow water, insects and pollen to cross the border.
The US Homeland Security Secretary, Michael Chertoff, has said that the fencing is needed. He has rejected arguments that the Rio Grande provides an adequate barrier as water levels in the river often drop, allowing people to wade across.
An estimated 12 million illegal immigrants are in the US, where attempts at a comprehensive overhaul of immigration law have repeatedly stalled in Congress.