The U.S. border patrol told The Associated Press that detentions - which it uses to judge illegal migration rates - jumped 25 percent to 535,000 in the six months ending March 31 compared to a year ago.
Near Sasabe, a town bordering the Arizona desert that's the busiest illegal border crossing area, an average 2,000 people arrive daily.
On a recent day, at a break in a barbed-wire fence outside Sasabe, about 300 migrants scrambled out of 10 trucks and four vans within 30 minutes with their smugglers, who led crowds along a worn trail. As the sun set, they disappeared into rolling hills that hide the treacherous desert.
Raudel Sanchez, a 22-year-old farm worker, said he wanted to get back to his job at a Minnesota ranch.
Sanchez crossed into the United States through Sasabe three years ago, but says the journey is getting more difficult. He walked three days in the desert and was out of water when he was caught in Arizona and deported.
Undeterred, he said he planned to take a bus to Altar, a northern city about 70 miles from the border where migrants hire smugglers. From there, he planned to head back to Sasabe and cross again.
"It's already very hard to cross, but it's going to be even harder," he said in Nogales. "I need to try again, at least one more time, and if I fail, I'll go back home."
Many migrants are betting on the approval of Bush's migration proposal, which faces an uphill battle in Congress. About 75 percent of those arrested are Mexican, while the rest are from Central America and other places, U.S. customs officials said.
In January, Bush proposed a guest-worker plan that would give legal status to undocumented migrants already working in the United States and to those outside the country who can prove they have been offered a job.
Because it's hard to get a job offer while in Mexico, many are heading north now, hoping to get settled before a program is in place.
Mark Krikorian, executive director for the Center for Immigration Studies, a group that favors stricter immigration policies, said the rise in illegal migration also shot up in 1986 when an amnesty was announced.
"Illegal aliens will respond to the messages the government sends," Krikorian said. "When we send the message that we are thinking about amnesty, they decide it may be worth it to try to cross."
Illegal migration had been declining along the U.S.-Mexico border since 2000. U.S. border patrol figures show detentions dropped from 1.6 million in 2000 to 905,000 in the fiscal year that ended last Sept. 30.
There is no exact data on the number of people crossing illegally. But in an indication of increased traffic, 535,000 illegal migrants were arrested along the U.S.-Mexico border from Oct. 1 to March 31, said Gloria Chavez of the U.S. Customs and Border Protection Bureau.
In the same period, the border patrol's Tucson sector detained 70,000 more people, an increase of 49 percent.
U.S. Customs and Border Protection Commissioner Robert Bonner attributes part of the jump to increased security. "The main reason we're seeing an increase in apprehensions is because the border patrol is more effective, particularly in the Tucson sector," he said.
But Mexican officials are also seeing an increase. Grupo Beta, a Mexican government-sponsored group that tries to discourage migrants from crossing and aids those stranded in the desert, said 56,000 migrants went through Sasabe in March compared to 41,000 in
In Altar, a farming town that has become the gathering point for those heading to Arizona, street vendors sell backpacks, water jugs and salt pills by the thousands.
The modest homes around the plaza, crowded with triple-decker bunk beds, serve as makeshift motels for migrants. They're almost always at capacity, said Francisco Garcia, a former mayor who now volunteers at the town's only migrant shelter.
"We're a town with a population of 6,000, and there have been weeks when we have twice as many people," Garcia said.
Under new security measures, about 300 more U.S. border agents will be deployed by June 1 along the Mexico-Arizona border. The number of border agents assigned to the Tucson sector will eventually increase from 1,800 to 2,500, Bonner said.
Many of the additional agents already have been sent to the Tohono O'odham Indian reservation, an area west of Sasabe where illegal migrant traffic has ballooned, said border patrol spokesman Charles Griffin.
The heightened border security is driving more migrants to more treacherous desert routes between Sonoyta and San Luis Rio Colorado in western Arizona, said Enrique Enriquez, an agent with Mexico's Grupo Beta.
"Migrants are telling us they're crossing through Sonoyta because there are fewer border patrol agents there," Enriquez said. "But with the new changes, we expect more migrants will start going west of there," where there are even fewer roads and people to help stranded migrants.
Grupo Beta plans to assign rescuers to Sonoyta in May, Enriquez said. Every year, hundreds of migrants die in the desert, where temperatures soar above 100 degrees in summer.
By Olga R. Rodriguez