President Donald Trump said he was holding off on declaring a state of emergency to end the partial United States government shutdown that dragged into a 23rd day on Sunday, as he insisted on $5.7 billion to build a Mexico border wall that congressional Democrats oppose.

Asked by Fox News why he didn't immediately declare a national emergency to secure the funds without congressional approval, Trump said he wanted to give opposition Democratic lawmakers more time to strike a deal.

"I want to give them the chance to see if they can act responsibly," he told Fox in an interview late on Saturday.

The US government shutdown became the longest on record at midnight on Friday, when it overtook a 21-day stretch in 1995-1996 under president Bill Clinton.

Trump fired off a series of tweets on Saturday in an effort to defend his stance and goad Democrats to return to Washington and end what he called "the massive humanitarian crisis at our Southern Border".

"Democrats could solve the Shutdown in 15 minutes!" he said in one tweet, adding in another, "We will be out for a long time unless the Democrats come back from their 'vacations' and get back to work. I am in the White House ready to sign!"

But most lawmakers left town on Friday and will not return before Monday, leaving little chance for any solution to the stalemate before then.

The impasse has paralyzed Washington — its impact felt increasingly around the country — with the president refusing to sign off on budgets for swaths of government departments unrelated to the dispute.

As a result, 800,000 federal employees — including FBI agents, air traffic controllers and museum staff — received no paychecks on Friday.

Retreat from 'emergency'

At a White House meeting on Friday, Trump described an emergency declaration as the "easy way out," and said Congress had to step up to the responsibility of approving funding for the wall.

"If they can't do it ... I will declare a national emergency. I have the absolute right," he said.

Trump, however, acknowledged that such a move would likely trigger a legal battle ending in the Supreme Court.

Opponents say such a unilateral presidential move would be constitutional overreach and set a dangerous precedent in similar controversies.

'Under siege'

Trump pushed back on Saturday on a media report that his White House was "chaotic" with no plan or strategy to end the shutdown.

To understand the plan "you would have to understand the fact that I won the election, and I promised ... a Wall at the Southern Border. Elections have consequences!" he tweeted.

Both Democrats and Republicans agree that the US-Mexican border presents challenges, but Trump has turned his single-minded push for more walls into a crusade that opponents say is a stunt to stoke xenophobia in his right-wing voter base.

For the president, who visited the Texas border with Mexico on Thursday, the border situation amounts to an invasion by criminals.

Only in recent days has he begun describing the problem as "humanitarian". Some studies show that illegal immigrants commit fewer crimes than people born in the United States.

And most narcotics are smuggled through heavily guarded checkpoints in vehicles, the government's Drug Enforcement Administration said in a 2017 report.