Nielsen told Fox News host Laura Ingraham that 1,600 Guard members are currently on the frontier, a number that will next rise to 2,200 as their duties expand.

"Originally we were focused on border patrol support but now we've extended it," with troops also supporting ports of entry as well as intelligence and analysis, Nielsen said.

"It's a huge force multiplier," she said.

US President Donald Trump, who has cracked down on illegal immigration, said in April he would send thousands of National Guard soldiers to the southern border, where they could remain until his promised border wall is constructed.

Trump's order could eventually see around 4,000 Guardsmen -- under the control of their respective state governors -- along the border, which spans four US states.