US National Guard troops have been deployed along the country's southern border between the state of California and Mexico.
They are the first of more than 1,000 troops President Barack Obama has authorised to beef up border security.
There are some 12 million people in the US illegally and most have crossed the southern border from Mexico.
Immigration has become a central issue ahead of important national elections in November.
California has sent more than 200 military personnel to its southern border with Mexico.
They are the first of up to 1,200 National Guard Troops who will support existing border agents in the four US states along the country's southwestern frontier.
California's Governor Arnold Schwarzenegger said the troops had been called to help protect the safety of the American people. They will be operational from September.
But he warned the country must find a permanent solution to its broken immigration system.
The deployment of National Guard troops is part of Mr Obama's response to loud calls to do more to secure America's borders against drug smugglers, human traffickers and illegal immigrants.
Another border state, Arizona, recently passed tough new anti-immigration laws, complaining the federal government had failed to act.