A partial government shutdown in the United States over President Donald Trump's demand for $5.7bn to build a wall along the US-Mexico border has entered a record 22nd day.

Trump, holed up in the White House with Congress adjourned for the weekend, warned of a much lengthier impasse and blamed the Democrats, as Saturday marked the longest shuttering of federal agencies in the US history.

The president said he had "no idea" whether he can get a deal with House speaker Nancy Pelosi, in an interview with Fox News on Saturday.

Pelosi, a Democrat, opposses spending money on what she calls an "ineffective, wasteful wall".

"We will be out for a long time unless the Democrats come back from their 'vacations' and get back to work," he tweeted.

We have a massive Humanitarian Crisis at our Southern Border. 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! — Donald J. Trump (@realDonaldTrump) January 12, 2019

Democrats say Trump shut the government in a "temper tantrum" by refusing to sign a bipartisan funding legislation last year that did not include money for his wall.

National emergency

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Faced with Congressional opposition, the president has threatened to declare a national emergency in order to secure funds for the wall.

"I have the absolute right to declare a national emergency," Trump told Fox News.

He said he hasn't declared the emergency because he's giving Congress a chance to "act responsibly."

"If they don't come to their senses, I'll do it. You know what I'll do? I'll do a national emergency we're all set, it's 100 percent and I'll have to do that."

The closure, which began on December 22, broke a decades-old record by a 1995-1996 shutdown under former President Bill Clinton that lasted 21 days.

Federal workers affected missed their first paycheques on Friday, heightening concerns about mounting financial pressures on employees, including air traffic controllers and airport security officials who continue to work without pay.

Concerns over pay, depression

Roughly 800,000 federal workers did not receive paycheques that would have gone out on Friday. Some have resorted to selling their possessions or posting appeals on online fundraising sites to help pay their bills.

Miami international airport in Florida said it will close one of its terminals over the next several days due to a possible shortage of security screeners, who have been calling in sick at twice the normal rate.

A union that represents thousands of air traffic controllers sued the federal aviation administration on Friday, saying it had violated federal wage law by failing to pay workers. It is at least the third lawsuit filed by unions on behalf of unpaid workers.

The head of the US Secret Service, which is responsible for protecting Trump, has warned employees that financial stress can lead to depression and anxiety.

"Keep an eye out for warning signs of trouble," Director R D "Tex" Alles wrote in a memo seen by the Reuters news agency.

The transportation security administration, responsible for airport security screening, said its rate of unscheduled absences rose to 5.6 percent on Saturday from 3.3 percent a year ago but that security standards have not been compromised.

To support workforce, authorities said they were processing pay for employees who worked on the first day of the shutdown and announced $500 bonuses for uniformed screening officers.

Trump is considering a possible national emergency declaration that would end the shutdown and allow him to obtain his wall funding by circumventing Congress. But on Friday, he said he would not take such a step "right now".

'Tell them to get it done!'

Trump also urged his 57.2 million Twitter followers to contact Democratic legislators and "Tell them to get it done!"

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Democrats, who say the wall is an ineffective, outdated answer to a complex problem, have passed several bills in the House of Representatives to reopen the government without funding for Trump's barrier.

But the legislation has been ignored by the Republican-controlled Senate.

Trump originally pledged Mexico would pay for the wall, which he says is needed to stem the flow of illegal immigrants and drugs. But Mexico has refused.