Mr Chertoff said the system has already helped catch smugglers

The $20m 'virtual fence' already covers 28 miles (48km) of the border between Arizona state and Mexico.

The system has already helped catch smugglers, and would be deployed elsewhere, said US Homeland Security Secretary Michael Chertoff.

But he said plans to complete 770 miles (1,130km) of physical fence remain.

"I have personally witnessed the value of this system," said Mr Chertoff.

"I have spoken directly to the border patrol agents... who have seen it produce actual results in terms of identifying and allowing the apprehension of people who are illegally smuggling across the border."

Unmanned towers

Built by Boeing, the virtual fence is part of a strategy to stop illegal immigrants as well as drug-smugglers attempt to pass into the US on foot or in vehicles.

Its technology - including 100-ft (30-metre) unmanned surveillance towers equipped with sophisticated sensor devices - is capable of distinguishing people from cattle at a distance of about 10 miles (16km).

The virtual fence targets illegal immigrants and drug smugglers

The system's cameras and radars are powerful enough to determine whether people are carrying backpacks that may contain weapons or drugs.

The US government plans to extend the technology to other areas of the Arizona border, as well as sections of Texas, possibly within months.

In a televised debate in Texas on Thursday, both Democratic party presidential candidates, Barack Obama and Hillary Clinton, said high-technology surveillance could lessen the need for a physical barrier.

A highly charged political issue, immigration has been at the forefront of this year's presidential campaign.

Plans for the physical barrier covering about a third of the US-Mexico border have drawn fierce criticism.