Trump ignited a political firestorm and sent a chill through the United States' European allies on Saturday when he said he would "encourage" Russia to attack any NATO country that does not meet financial obligations.

"Let's be serious. NATO cannot be an a la carte military alliance, it cannot be a military alliance that works depending on the humour of the president of the US" day to day, Borrell said.

"It exists or it not exists," he said, adding that he was not going to keep commenting on "any silly idea" emerging from the US presidential election campaign.

The words from Trump -- who, when in power, pulled America out of an international pact designed to curb Iran's nuclear programme, and the Paris climate accord -- prompted a broadside from current US President Joe Biden.

Biden called Trump's comments "appalling and dangerous".

NATO chief Jens Stoltenberg warned Sunday that "any suggestion that allies will not defend each other undermines all of our security, including that of the US".

Trump, 77, is all but assured the Republican nomination for the White House in the November vote against Biden, 81.