WASHINGTON - US President Donald Trump's conflict with Democrats over a partial government shutdown escalated on Thursday with Trump denying House Speaker Nancy Pelosi use of military aircraft for an overseas trip in apparent retaliation for her suggestion that he delay the annual State of the Union address.

The rising tension suggested that the shutdown, now in its 27th day, remains far from being resolved.

In the meantime, the Trump administration continued to try to mitigate the shutdown's impact. The State Department said it would call its furloughed employees back to work next week for national security reasons.

Pelosi had been scheduled to travel on a military aircraft as part of a congressional delegation to Belgium, where NATO is based, and Afghanistan to visit American troops stationed there.

In a letter to Pelosi denying her delegation the use of a plane, the Republican president called the trip an "excursion" but the Democrat's spokesman said the trip to the Afghanistan war zone was for security and intelligence briefings.

"In light of the 800,000 great American workers not receiving pay, I am sure you would agree that postponing this public relations event is totally appropriate," Trump wrote.

He told Pelosi she could make the trip by flying commercial.

Trump's intervention stopped the trip just as Pelosi and other lawmakers were about to travel.

The president later canceled his own administration's plans to attend the World Economic Forum conference in Davos, Switzerland. Treasury Secretary Steven Mnuchin and Secretary of State Mike Pompeo had planned to attend after Trump canceled his own participation at Davos.

White House Press Secretary Sarah Sanders said that in forcing Pelosi not to travel on a US government jet, Trump had acted within his authority as commander in chief.

"He postponed her ability to use military air, which must be approved" by the Department of Defense, Sanders said.