NOGALES, Ariz. >> A group of migrants was hustling north through the southern Arizona desert the other night when one of their cellphones vibrated with a text message. “Watch out,” it warned. “Things are hot up ahead. Take cover in the bushes.”

The message, signaling the presence of the Border Patrol, was sent by a smuggler watching the group’s progress through binoculars from a hillside on the Mexican side of the border, members of the group said later. It was part of what border officials and immigrant activists say is an emerging trend in illegal border crossing — the use of what is being called the cybercoyote.

“I’ve crossed eight times and this is the first time they’ve directed me with my cellphone,” said Sandra Silva, 30, a native of Guadalajara, Mexico, who was on her way to Phoenix. “It’s like a guide through the desert.”

Increased enforcement has made it difficult to sneak into the United States, officials say. And repeat offenders caught in the act are more often receiving stiff prison terms, making smugglers more cautious about risking arrest themselves.

Guides still accompany the bulk of the migrants crossing the border, activists and Border Patrol agents say. Those guides are in regular radio contact with confederates, who warn of trouble ahead. But the Border Patrol has been noticing cases of migrants crossing alone but in cell contact with guides, said Mario Escalante, a spokesman for the Tucson office of the Border Patrol.

Mobile phones are ubiquitous in Mexico; many migrants consider them essential when crossing, right up there with sturdy shoes and jugs of water.

Now, though, in addition to using cellphones to keep relatives up to date on their progress, some illegal immigrants rely on them to keep out of the reach of the authorities. Silva said her group had no coyote with them but had received directions by text.

The messages typically come during migrants’ first hour or so of hiking north, those who had used the new system said. If they make it that far, the illegal immigrants then meet up with guides on the United States side, who help them trek further north to waiting vehicles.

Aiding the process are numerous spotters, who monitor the southern Arizona desert from lookout points and help steer the migrants, as well as drug shipments, away from the authorities.

Smugglers are constantly innovating to elude the authorities, veteran Border Patrol agents say. “They always come up with new, clever ways of trying to avoid us,” said T.J. Bonner, past president of the National Border Patrol Council, the union for agents. “This way minimizes the risk to smugglers and you’re using a technology that is relatively cheap.”

To reduce the number of fatalities among border crossers, a University of San Diego professor, Ricardo Dominguez, has been developing a cellphone application to help guide illegal immigrants to water stations and other points of safety.

His project outraged three Republican members of Congress, who wrote to university officials last year condemning the research and suggesting that he may be violating the law by encouraging illegal immigration.

Border Patrol agents have complained that a lack of coverage complicates their ability to communicate. On some stretches where coverage is not a problem, the Border Patrol has urged residents to report suspicious activity via text message.

After a rancher was killed along the border in 2010 in a high-profile case that remains unsolved, Rep. Gabrielle Giffords pushed to improve cellphone coverage in the region. After Giffords, a Democrat, was shot on Jan. 8, a Republican colleague from Texas, Rep. Ted Poe, introduced legislation she had supported to use federal grants to help beef up communications along the border.

“It was very obvious to me during my recent visit to southern Arizona that there are too many areas where cellphones simply do not work,” Poe said in a statement in March.

© 2011 The New York Times Company