Mexico City: Mexico on Monday called on the United States to alter a plan to expand border fences designed to stem illegal immigration, saying the barriers would threaten migratory species.

Ways of minimising environmental damage from the fences could include the creation of cross-border bridge areas so that ecosystems remain connected and "green corridors" of wilderness without roads that would be less attractive to smugglers, according to a report released on Monday and prepared for the Mexican government by experts and activists from both nations.

The report also suggested "live" fences of cactuses, removable fencing, and more permeable barriers to allow water, insects and pollen to cross the border.

Ecologists say among the species affected would be Mexican jaguars and black bears, and the endangered, antelope-like Sonora Pronghorn.

Mexico's Environment Secretary Juan Rafael Elvira said on Monday: "The eventual construction of this barrier would place at risk the various ecosystems that we share." The 3,218-kilometre border was not just desert, but included mountains, rivers and wetlands, Elvira observed.