To stem the continuing flow of illegal immigrants over the Mexican border, the United States should build 200 miles of fences and obstacles costing $500 million a year, an immigration group said Thursday.

A report by the Federation for American Immigration Reform called for double fences, concrete barriers and secure metal fences at major points of illegal entry such as San Diego, El Paso, Texas, and Tucson, Ariz., where 90 percent of illegal crossings occur."Half measures will avail us nothing," said the 89-page report, prepared by former Border Patrol chief Roger Brandemuehl. "If the United States is serious about enforcing its immigration laws, it will have to erect fencing that is difficult to climb and much more difficult to cut through."

The barriers would expand on and improve 28 miles of chain-link fencing in states along the border that has proved easy to penetrate.

Nearly 1 million people were caught trying to illegally cross the Mexican border into the United States last year, but between 2 million and 3 million made it through, according to the Immigration and Naturalization Service.

The INS announced plans Wednesday to dig a four-mile, $2 million ditch in California, across from Tijuana International Airport, to stop smugglers of aliens and drugs.

The group's report, which calls itself the oldest and largest immigration reform organization in the United States, also called for expanded use of electronic sensors, lighting, dogs, imaging devices, helicopters, all-terrain vehicles and horses.

It said a metal fence along much of the 200 miles where illegal entry is concentrated would be tough to climb and to cut through. But along the border near San Diego and El Paso, where the heaviest influx of aliens occurs, double fences and concrete and earth barriers would be required, the report said.

*****

`Ditch' raises Mexico's ire

Mexico's government reacted with surprise to a U.S. announcement that the main purpose of a planned 4-mile ditch along the border will be to deter drug smugglers and illegal immigrants, saying it was told the ditch was for drainage problems. In a statement Thursday, Foreign Ministry officials acknowledged they knew about plans for the 5-foot-deep, 14-foot-wide ditch along the San Diego-Tijuana border. But they said they had been told it was to solve rainwater drainage problems on the flat Otay Mesa, along the international border. Several Mexican congressmen reacted angrily to the proposal, calling it "unfriendly" and saying it would not stop the flow of illegal immigrants.