President Trump has made a historic threat to congress.

President Donald Trump has threatened to keep the US government partially shut for months or years after he and Democratic leaders failed to resolve their dispute over Trump’s request for $US5.6 billion ($A7.9 billion) to build a wall on the Mexican border.

After Democratic congressional leaders refused Trump’s requests at a meeting in the White House Situation Room, the Republican president threatened to take the controversial step of declaring a national emergency and building the wall without congressional approval.

Trump is withholding his support for a bill that would fully fund the government until he secures money for the wall. As a result, around 800,000 public workers have been unpaid, with about a quarter of the federal government closed for two weeks.

Senate Democratic Leader Chuck Schumer said Democrats had told Trump during the meeting to end the shutdown. “He resisted,” Schumer said. “In fact, he said he’d keep the government closed for a very long period of time, months or even years.”

Trump confirmed that comment but painted a more positive picture of the meeting, the first since a new era of divided government began when Democrats took control of the House of Representatives on Thursday.

“We had a very, very productive meeting, and we’ve come a long way,” Trump said.

According to a source familiar with the White House discussion, Trump opened the meeting with a speech that lasted at least 15 minutes in which he insisted on the need for billions of dollars to fund a border wall.

The source also said Trump brought up recent impeachment threats during those remarks, arguing that he had notched a strong performance as president and should not be a target for impeachment.

The president later told reporters that Nancy Pelosi, the new Democratic speaker of the House of Representatives, said Democrats were not looking to impeach him.

Raising the stakes in his tussle with the newly emboldened Democrats, Trump threatened extraordinary measures to build the wall, which he says is needed to stem the flow of illegal immigrants and drugs into the United States.

A reporter asked Trump whether he had considered declaring a national emergency to build the wall.

“Yes, I have. And I can do it if I want,” Trump said. “We can call a national emergency because of the security of our country … I may do it. But we can call a national emergency and build it very quickly. And it’s another way of doing it. But if we can do it through a negotiated process, we’re giving that a shot.”

Emergency powers have been invoked by previous US presidents during times of war.

Senator Jack Reed, the senior Democrat on the Senate Armed Services Committee, criticised the comments, saying in a statement, “Declaring a trumped up national emergency in order to skirt congressional approval is wrong.”

The US Constitution assigns Congress the power to fund the federal government, so Trump likely would face legal challenges if he tried to bypass Congress on financing the wall. Building a wall – and having Mexico pay for it – was one of Trump’s main promises when he ran for president in 2016.