Trump pushes border security as he tours California fence

US President Donald Trump consulted with immigration agents in a California border city on Friday, while insisting that the nation's immigration system was overburdened and declaring that "our country is full".

The political battle over the border ignited again just as Air Force One touched down in Calexico, not far from Mexico, as California and 19 other states that are suing Trump over his declaration requested a court order to stop money from being diverted to fund the project.

But Mr Trump, who has placed hard-line immigration policy at the heart of his administration, declared that his move, which included vetoing a congressional vote and some opposition from his own party, was necessary.

"There is indeed an emergency on our southern border," Mr Trump said at a border security briefing, adding that there has been a sharp rise in illegal crossings.

"It's a colossal surge, and it's overwhelming our immigration system. We can't take you any more. Our country is full."

Although the wall – his signature campaign promise – remains unbuilt, Mr Trump declared that at least 400 miles of the border barrier would be erected over the next two years even as he tried to blame Democrats for a lack of progress on it.

Donald Trump
The fence that Mr Trump is touring is a two-mile section that was a long-planned replacement for an older barrier, rather than new wall (Jacquelyn Martin/AP)

"The crisis is a direct result of the obstruction by Democrats," the president claimed.

Mr Trump also denied that he changed his mind about shutting down the border with Mexico, a threat he backed off from on Thursday.

Mr Trump said he reversed course because he saw Mexico get tougher in stopping an influx of immigrants from moving north.

"Mexico has been absolutely terrific for the last four days," the president said as he left the White House.

"I never changed my mind at all. I may shut it down at some point."

Though Mr Trump, who has pulled a series of about-faces in recent days, walked away from his threat to close the border, he went ahead and highlighted the conditions at the boundary with Mexico.

The fence that Mr Trump is touring is a two-mile section that was a long-planned replacement for an older barrier, rather than new wall.

The White House says the barrier is marked with a plaque bearing Mr Trump's name and those of top homeland security officials.

Mr Trump, who wants to return to illegal immigration as a key 2020 re-election issue, also used Twitter earlier Friday to claim that he could revive his threat to shut the border, a move that fellow Republicans warned would have a devastating economic impact.

"If for any reason Mexico stops apprehending and bringing the illegals back to where they came from, the US will be forced to Tariff at 25% all cars made in Mexico and shipped over the Border to us. If that doesn't work, which it will, I will close the Border," Trump tweeted.