MEXICO CITY -- Mexico is calling on the United States to alter a plan to expand border fences designed to stem illegal immigration.

Full Immigration Coverage

A new report prepared for the Mexican government said the barriers would threaten migratory species accustomed to roaming freely across the frontier.

The U.S. could consider creation of "cross-border bridge areas" for wildlife, adding that such "green corridors" of wilderness without roads would be less attractive to smugglers.

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

Ecologists said species affected include Mexican jaguars and black bears, and the endangered, antelope-like Sonora Pronghorn.

The report comes at a time whwen Republican senators are drafting legislation that would provide $3 billion in emergency funding to beef up border security -- including more walls.

Republican Sen. Judd Gregg, one of the co-sponsors of the amendment, said it aims to establish operational control over the U.S.-Mexico border within two years.

The money would go to hiring and training 23,000 Border Patrol agents, and provide four remote-contyrolled drone aircraft, 105 ground-based radar and camera towers, 300 miles of vehicle barriers and 700 miles of border fencing, Gregg said.

What this amendment does is acknowledge the fact that what we have here is an emergency. It is as big and as important an emergency relative to national security as the war in Iraq," Gregg said.

"Nothing is more important to us from a standpoint of protecting our national security than making sure that we get operational control over the borders and as this amendment moves forward to do," he said.

The White house has already threatened a veto of the underlying homeland security bill for breaking Bush's budget and Gregg said the White House opposes the border security plan offered by Senate Republicans.

"The administration position ... is that they oppose it," Gregg said.

During the immigration debate last month, proponents of the broader approach such as Bush and Sen. Lindsay Graham of South Carolina argued that splitting up the compromise immigration package and passing border enforcement first would doom the broader effort.