The US has deployed a first contingent of National Guard troops along its border with Mexico.
The soldiers will take part in construction and other work
This is part of an operation aimed at curbing illegal immigration.
Some 55 soldiers from the Utah National Guard took up positions along border points with Mexico. They will be joined by 6,000 more in the coming months.
The issue is dividing the US, provoking fierce debate in Congress between Republicans and Democrats on how stringent the measures should be.
The soldiers' job is to help secure the border rather than arrest immigrants, and to free up border agents to catch illegal immigrants.
"Our mission here is to provide a structure to support the border patrol," Maj Hank McIntyre told the AFP news agency.
They will help to extend a border fence and construct a new road to be used by border patrols.
"Our main task is construction. They're working on three different construction projects," Maj McIntyre said.
The governor of the state of Washington, Chris Gregoire, says she will not send any of her state's National Guard troops unless they choose to serve there.
The governor of California, Arnold Schwarzenegger, only agreed to commit troops after the federal government said it would carry the full cost of their deployment.
The deployment, named Operation Jump Start, was announced by President Bush in May as part of his immigration reform plan.
Measures include extending a fence along the Mexican border and increasing the number of patrols along it.
They are part of a package approved recently by the US Senate.
The package also contains provisions for a guest-worker programme and for offering citizenship to some illegal immigrants.
Another, tougher immigration bill has been backed by the House of Representatives, which calls for all illegal immigrants to be criminalised.
The Senate bill will need to be reconciled with the tougher House of Representatives bill to be passed into law.
Right-wing groups have protested against illegal immigrants, while millions of people marched in support of them last month.