President Donald Trump gave his strongest indication yet Friday that he will soon declare a state of emergency, bypassing the need for congressional approval to fund a controversial US-Mexico border wall.

Trump hinted in remarks at a White House meeting on cross-border trafficking that a declaration - which would further heat the political temperature around the issue - could even come in his State of the Union speech to Congress next Tuesday.

"Well, I’m saying listen closely to the State of the Union, I think you’ll find it very exciting," Trump said. 

"I’m certainly thinking about it," he said of declaring the emergency. "I think there’s a good chance we’ll have to do that."

Trump's threat comes well before the expiration of a February 15 deadline that he set for Congress to agree on funding for wall construction. But on Thursday he described negotiations with opposition Democrats "a waste of time."

The president says that existing sections of walls should be extended along the border to stop what he describes as an uncontrolled invasion of criminals.

Democrats, who control the lower house, have repeatedly rejected Trump's funding demands, saying that he has made the wall project a political crusade to demonise immigrants and to satisfy his base.

He previously tried to pressure Congress into backing his idea by refusing to sign off on budgets for swaths of the federal government, leading to a five-week shutdown of some 800,000 government jobs.

By declaring a national emergency on the border, Trump would in theory be able to tap unrelated funding to use wall construction. However he would almost certainly face immediate court challenges, with Democrats arguing that there is no real emergency on the frontier.