President Bush has launched a drive to halt illegal immigration across America’s porous southern border, amid growing fears that terrorists may be using Mexico as a base camp before heading to Arizona, Texas, and California.

A string of alarming incidents has convinced Bush administration officials that lax immigration rules, designed to cope with the huge numbers of illegal entrants from Mexico, have become a significant loophole in the war on terror.

Over the past month, border agents from Arizona and Texas have anonymously reported recent encounters with dozens of Arab men who have made their way across the 2,000-mile Mexican border.

Patrol agents told one Arizona newspaper that 77 males “of Middle Eastern descent” were apprehended in June in two separate incidents. All were trekking through the Chiricahua mountains and are believed to have been part of a larger group of illegal immigrants. Many were released pending immigration hearings. According to Rep. Solomon Ortiz, a Democrat of Texas, similar incidents are “happening all over the place. It’s very, very scary.”

The two groups of Arab males were discovered by patrol guards from Willcox, Ariz. “These guys didn’t speak Spanish,” said one field agent, “and they were speaking to each other in Arabic. It’s ridiculous that we don’t take this more seriously. We’re told not to say a thing to the media.” A colleague told the paper: “All the men had brand-new clothing and the exact same cut of mustache.” Local ranchers have also reported a rise in the sightings of large groups of young males.

Last month, border patrol agents at McAllen-Miller International Airport in Texas arrested a woman believed to be Pakistani, who was carrying a false South African passport. The woman, Farida Ahmed, is still being questioned by the FBI. She was traveling to New York and admitted to having illegally crossed the Mexican border.

More than 1.2 million people attempt to cross American borders illegally each year. The vast majority are Mexicans, who are immediately repatriated. But under existing laws, which take into account limited detention facilities, many non-Mexican illegal immigrants are released prior to a hearing with an immigration judge, which most fail to attend. Almost 22,000 non-Mexican immigrants have been released pending a hearing since last October. Once released, they are free to travel on their own throughout America. According to one Texas immigration official, those who slip through the net come “from all over the world.”

The possibility of a southern border loophole for potential terrorists was recognized as early as last year.

According to new laws, approved by President Bush and announced last week by the Homeland Security Department, illegal immigrants who are not Mexican will be repatriated within days, without a hearing. The effort comes in response to a prolonged campaign by Texas congressmen, who have argued that the president’s home state was in danger of becoming a gateway to America for terrorists.