LAREDO, Texas (AP) -- A Mexican army convoy began crossing into the United States on Thursday to bring aid to victims of Hurricane Katrina.

Carrying water treatment plants and mobile kitchens that can feed 7,000 people daily, the convoy bound for San Antonio is the first Mexican military unit to operate on U.S. soil since 1846.

The first green tractor-trailers, with Mexican flags attached to the tops of their cabs, crossed the international bridge at Laredo at about 8:15 a.m.

The rest of the 45-vehicle convoy was in a staging area on the U.S. side in about 15 minutes.

The convoy will be escorted by the U.S. Army and the Texas Department of Public Safety.

It was scheduled to leave after the leader of the convoy, Gen. Francisco Ortiz Valadez, greeted the head of the U.S. Army unit in charge of the escort, Brig. Gen. F. Joseph Prasek.

Military engineers, doctors and nurses are among the 200 people headed to San Antonio.

The Mexican government was already planning another 12-vehicle aid convoy for this week.

It has sent a Mexican navy ship toward the Mississippi coast with rescue vehicles and helicopters.

Mexico has sent disaster relief aid missions to other Latin American nations, but not to the United States.

In 1846, Mexican troops briefly advanced just north of the Rio Grande in Texas, which had then recently joined the United States.

Mexico, however, did not then recognize the Rio Grande as the U.S. border.

The two countries quickly became mired in the Mexican-American War, which led to the loss of half of Mexico's territory in 1848.

Copyright 2005 The Associated Press.