US Secretary of Defense Pete Hegseth's plane was forced to make an unexpected stop in the UK when he was flying to the US from Brussels because of a crack in the plane's windshield.

"The plane landed based on standard procedures and everyone onboard, including Secretary Hegseth, is safe," Pentagon spokesman Sean Parnell said.

After Hegseth left Brussels after attending a NATO Defense Ministers meeting, open source flight trackers spotted his aircraft lose altitude and begin broadcasting an emergency signal.

Hegseth earlier in the day warned that the US would "impose costs" on Russia if it continued to oppose US President Donald Trump's push to end the war in Ukraine.

In February, an Air Force plane carrying Secretary of State Marco Rubio was similarly forced to return to Washington after an issue with the cockpit windshield.