A spokesman for Pelosi, Drew Hammill, said the trip to Brussels was intended "to affirm the United States’ ironclad commitment to the NATO alliance."

The Afghanistan leg of the trip would have obtained "critical national security and intelligence briefings from those on the front lines," Hammill said.

A House Democratic aide, speaking on condition of anonymity, told Reuters that details of the trip were not supposed to be revealed for security reasons, as is customary. The White House, however, made the letter to Pelosi detailing the itinerary public.

A US defense official, speaking on condition of anonymity, said Pelosi had requested and been granted Defense Department support for overseas travel, although the president has the authority to cancel Pelosi's travel.

At the Capitol, members of the delegation had already boarded a bus bound for Joint Base Andrews just outside Washington, from which the plane was set to depart, when news of Trump's decision broke.

It appeared to be a response to a letter Pelosi sent to Trump on Tuesday, suggesting he delay the annual State of the Union address to Congress, scheduled for Jan 29, to some point after the government reopens.

Pelosi cited concerns over the capacity of the Department of Homeland Security and the Secret Service to safeguard the Capitol during the event. The agency later said it was capable of securing the venue.

Trump's action drew criticism from one of his Republican allies in the Senate. "One sophomoric response does not deserve another," Republican Senator Lindsey Graham wrote on Twitter.

He criticized Pelosi's move on the State of the Union as "blatantly political" but said, "President Trump denying Speaker Pelosi military travel to visit our troops in Afghanistan, our allies in Egypt and NATO is also inappropriate."

Democratic Representative Elaine Luria, a first-year lawmaker and a 20-year Navy veteran, had harsh words for Trump.

"The president’s comment that lawmakers visiting Afghanistan is a ‘public relations event’ is an insult to the brave men and women serving in harm’s way,” Luria said.

Also on Thursday, the State Department said it would recall its employees and take steps to pay them salaries - a departure from other agencies.

"As a national security agency, it is imperative that the Department of State carries out its mission," Deputy Under Secretary of State Bill Todd said in a statement. "We are best positioned to do so with fully staffed embassies, consulates and domestic offices."

Todd said the department's employees would be paid on Feb 14 for work performed beginning on or after this coming Sunday.

The department would review its available funds and "legal authorities" beyond the upcoming pay period to try to cover future payments, he said.

About one quarter of federal agencies have been shuttered since Dec 22, with Democratic lawmakers refusing to accede to Trump's demands to pay for a wall along the US border with Mexico.

Trump is holding out for $5.7 billion for a border wall.

Democrats, who took over the US House of Representatives this month, have rejected his demands, saying there are cheaper, more effective ways of enhancing border security.

Before leaving for the weekend, the House passed another bill that would fund the government through Feb. 28, but the vote was later vacated and held over until next week after a procedural objection from Republicans.

The Senate, however, has refused to bring any funding bill to the floor for a vote that Trump does not support.

Reuters