US President Donald Trump told House of Representatives Speaker Nancy Pelosi on Thursday that her trip to Belgium, Egypt and Afghanistan had been postponed over the partial government shutdown that has run into its 27th day.

It was not immediately clear what trip Trump was referring to. Politicians often do not publicise foreign trips before they occur for security reasons.

The US House speaker normally uses a military plane for overseas travel and it was on that basis that Trump, as commander-in-chief, appeared to be acting.

"Due to the Shutdown, I am sorry to inform you that your trip to Brussels, Egypt and Afghanistan has been postponed," Trump wrote in a letter to the top House Democrat.

"We will reschedule this seven-day excursion when the Shutdown is over," he said, adding that Pelosi could still go on the trip if she flew commercially.

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Trump's letter came a day after Pelosi sought to delay the president's State of the Union address, scheduled for January 29, due to the shutdown.

The country's two most powerful leaders appeared to be engaged in a game of constitutional one-upmanship, as negotiations to end the four-week stalemate failed to produce results.

Back-to-work order

More than 800,000 federal employees have been furloughed or required to work without pay since the shutdown began on December 22, after Trump refused to back down on his demand for more than $5bn in funding for a wall on the US-Mexico border. Democrats oppose the border wall, calling it ineffective, immoral and expensive.

On Wednesday, Trump signed legislation that guarantees workers will be paid once the shutdown ends, but many expressed anxiety over being able to pay their bills as the feud in Washington, DC continued.

On Thursday, the State Department instructed all US diplomats in Washington and elsewhere to return to work next week with pay, saying it had found money for their salaries at least temporarily despite the ongoing government shutdown.

In a notice to staff posted online and sent to employees, the department said it had found money to pay most of its employees beginning on Sunday or Monday for their next pay period. They would not be paid for time worked since the shutdown began in December until the situation was resolved, said the notice, which was signed by William Todd, the deputy undersecretary of state for management.

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It was not immediately clear where the money was found, but the department said it would use "existing funds as well as other available fiscal authorities to shift existing balances to restart payroll funding."

Salaries cannot be guaranteed beyond the next pay period, which ends on February 14, if the shutdown did not end by then, the department said. However, it said it would "review its balances and available legal authorities to see if other flexibilities may be available."

The department said it was taking the step because it had become clear that the lapse in funding was harming essential diplomatic and national security objectives.

"While the department has done its best to address matters essential to achieving US national security and foreign policy objectives during the ongoing lapse, it has become clear as the lapse has continued to historic lengths that we need our full team to address the myriad critical issues requiring U.S. leadership around the globe and to fulfill our commitments to the American people," it said.

It added that the department's leadership was "deeply concerned" about the financial hardships faced by its employees.

'Maybe, he thinks it's OK to not pay people'

Meanwhile, Pelosi said President Donald Trump has yet to respond to her request that he postpone his State of the Union address until the government was reopened so workers could be paid for providing security for the grand Washington tradition.

"We haven't heard - very silent," she told reporters on Thursday. "Let's get a date when government is open. Let's pay the employees. Maybe he thinks it's OK not to pay people who do work. I don't."

The president's planned January 29 address became a potential casualty of the four-week partial government shutdown after the Democratic leader cited concerns about whether the hobbled government could provide adequate security. Republicans cast Pelosi's move as a ploy to deny Trump the stage.

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Trump declined to address the stalemate over the speech on Thursday during a visit to the Pentagon, simply promising that the nation would have "powerful, strong border security".

The uncertainty surrounding the annual address also underscored the unraveling of ceremonial norms and niceties in Trump's Washington.

Pelosi reiterated she was more than willing to negotiate money for border security once the government was reopened, but she said Democrats remained opposed to Trump's long-promised wall, one of his signature campaign promises.

"I'm not for a wall," Pelosi said twice, mouthing the statement a third time for effect.

Pressure on Trump intensified, as lawmakers from both parties scrambled for solutions. The shutdown is already the longest ever.

While Trump's own advisers said the shutdown was proving a greater drag on the economy than expected, Trump showed no signs of backing off a fight that he views as vital for his core supporters.