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 Solomon Ortiz, the Congressman for Corpus Christi in 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, Arizona. "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 moustache." Local ranchers have also reported a rise in the sightings of large groups of young males.

Last month, border patrol agents at McAllen airport, 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 travelling to New York, and admitted to having illegally crossed the Mexican border. She was still carrying a pair of wet jeans in her travel bag.

More than 1.2 million people attempt to cross US borders illegally each year. The vast majority are Mexicans who are immediately repatriated. But under existing laws, which take 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 the United States. According to one Texas immigration official, those who slip through the net come "from all over the world".

"If you want to enter the US illegally," said the official, "the way to do it is to get to Mexico first." The possibility of a southern border loophole for potential terrorists was recognised as early as last year. In testimony before Congress, Steve McCraw, the assistant director of the FBI's Office of Intelligence, stated that "the ability of foreign nationals to use [the hearings procedure] to create a well-documented but fictitious identity in the United States, provides an opportunity for terrorists to move freely within the US without triggering name-based watch lists. It also enables them to board planes without revealing their true identity".

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.

In a letter to President Bush last week, Mr Ortiz wrote: "Law enforcement agencies across the south-west border are alarmed that the US is releasing thousands of OTMs [Other than Mexicans]. Those released include individuals from nations the US defines as state sponsors of potential terrorism, or from those nations that have produced large numbers of al-Qaeda militants."

Mr Ortiz also claims that immigration officials have privately warned him that a number of suspicious foreigners have been detained on the Mexican border and then released, including some who claimed to have travelled from South and Central America but were unable to speak Spanish.

The rules are to be tried out first in Tucson, Arizona, and Laredo, Texas. Any non-Mexican illegal immigrant found within 100 miles of the border, within 14 days of their arrival, will be immediately expelled. D'Wayne Jernigan, a sheriff in Del Rio, Texas, believes that the rule change should be immediately implemented across the entire Mexican border. Last month, immigration authorities told Sheriff Jernigan to release 17 Brazilians from his jail, before the FBI had located an interpreter in order to interview them.

"My concern is, are we serious about terrorism?" said Sheriff Jernigan. "Or about homeland security? Because we're turning loose non-Mexicans by the thousands. Entering this country illegally is a crime, and we're turning our heads and ignoring it."

See related story from KVOA-TV in Tucson, Arizona