President Donald Trump has made combating illegal migration a central part of his second term, and declared an emergency at the southern US border on his first day back in office.

"Beginning mid-July, we have begun working on establishing a detention center at Fort Bliss. Since then, work has begun for initial detainment capacity of 1,000 illegal aliens, with initial operating capacity likely to be achieved by mid-late August," Pentagon Press Secretary Kingsley Wilson told journalists.

"We will finish construction for up to 5,000 beds in the weeks and months ahead," she added.

It will not be the first time a US base has been used to hold migrants during Trump's presidency: he ordered the preparation of a 30,000-person "migrant facility" at the notorious Guantanamo detention camp in Cuba earlier this year, though it has not held anywhere close to that number of people.

The Trump administration's efforts to curb undocumented immigration have also included immigration raids, arrests and deportations on military aircraft.