"When today's meetings are over, I will be heading back to Washington," Trump tweeted.

"We won't be doing a press conference at the close of NATO because we did so many over the past two days. Safe travels to all!"

During his two days at the NATO summit near London, Trump has repeatedly engaged in lengthy question and answer sessions with reporters -- taking up so much time that his fellow leaders were caught on video mocking him.

But as the event drew to a close, he expressed anger at Canada's "two-faced" prime minister Justin Trudeau for the joking remarks and cancelled a previously arranged press briefing.