Asked about former U.S. President Trump's statement that he would "encourage" Russia to "do whatever the hell they want" to a NATO country that had not paid enough for defense, the top EU diplomat said the alliance cannot depend on the whims of the U.S. president.
"NATO cannot be an alliance that works depending on the humor of the president of the United States," Josep Borrell told reporters in Brussels on Monday.
"Let's be serious. NATO cannot be an 'à la carte' military alliance," he said, adding: "I'm not going to spend my time commenting on any silly idea that comes during this campaign in the US."
The EU and NATO are different bodies with different aims, but their memberships largely overlap.
On Saturday, Trump told a campaign rally in the state of South Carolina that when he was president, he had warned that he would not intervene in case of any Russian attack on allies that had not met their financial obligations to NATO.
During his time in office, Trump had a notoriously tense relationship with the U.S.' NATO allies, both in public, where he often berated traditional friends of the U.S. while praising its longtime foes, and in private, where he reportedly pushed to have the U.S. quit the alliance.