US President Donald Trump told NATO allies in a closed-door meeting on Thursday that governments needed to raise spending to two percent of economic output by January next year or the United States would go its own way, two people familiar with the discussions told Reuters.

The ultimatum was delivered in a session at the NATO summit, the sources said.

"He said they must raise spending by January 2019 or the United States would go it alone," one person said.

However, he did not directly threaten to withdraw formally from NATO, the people said.