Declaring "our country is full," Donald Trump on Friday insisted the US immigration system was overburdened and illegal crossings must be stopped as he inspected a refurbished section of fencing at the Mexican border.

The US president, making a renewed push for border security as a central campaign issue for his 2020 re-election, participated in a briefing on immigration and border security in Calexico before viewing a 2-mile see-through steel-slat barrier that was a long-planned replacement for an older barrier - and not new wall.

"There is indeed an emergency on our southern border," Mr Trump said at the briefing, adding that there had been a sharp uptick in illegal crossings. "It’s a colossal surge and it’s overwhelming our immigration system, and we can’t let that happen. ... We can’t take you anymore. We can’t take you. Our country is full."

As Air Force One touched down in the state, California and 19 other states that are suing Mr Trump over his emergency declaration to build a border wall requested a court order to stop money from being diverted to fund the project.

But Mr Trump, who ratcheted up his hard-line immigration rhetoric in recent weeks, declared that his move, which included vetoing a congressional vote, was necessary.