Top western officials criticised former president Donald Trump on Sunday after he suggested the US might not protect Nato allies who are not spending enough on defence from any Russian invasion.
"Any suggestion that allies will not defend each other undermines all of our security, including that of the US, and puts American and European soldiers at increased risk", said Nato Secretary General Jens Stoltenberg in a written statement.
"Any attack on Nato will be met with a united and forceful response."
Mr Stoltenberg was reacting to the remarks on Saturday by Mr Trump, who is likely to be the Republican nominee in this year's US presidential election.
US President Joe Biden criticised the comments.
"Donald Trump's admission that he intends to give Putin a green light for more war and violence, to continue his brutal assault against a free Ukraine, and to expand his aggression to the people of Poland and the Baltic States are appalling and dangerous," Mr Biden said on Sunday.
Polish Defence Minister Wladyslaw Kosiniak-Kamysz also weighed in.
"Nato's motto 'one for all, all for one' is a concrete commitment," Mr Kosiniak-Kamysz said on social network X.
"Undermining the credibility of allied countries means weakening the entire Nato.
"No election campaign is an excuse for playing with the security of the Alliance."
Germany's Foreign Ministry posted the message "One for all and all for one", with the hashtag #StrongerTogether on its English language X account after Mr Trump's comments.
EU Council President Charles Michel said: "Reckless statements on #Nato’s security and Art 5 solidarity serve only [Russian President Vladimir] Putin’s interest."
Article 5 of the Nato treaty says that an armed attack against an alliance member will be considered an attack against them all, causing collective self-defence.
Mr Trump, speaking during a political rally in South Carolina and appearing to recount a meeting with Nato leaders, quoted the president of "a big country" that he did not identify as asking, "Well sir, if we don't pay, and we're attacked by Russia, will you protect us?"
"I said: 'You didn't pay? You're delinquent?' He said: 'Yes, let's say that happened.' 'No I would not protect you. In fact I would encourage them to do whatever the hell they want. You gotta pay'."
EU Internal Market Commissioner Thierry Breton told France's LCI news channel: "We have heard that before … nothing new under the sun.
"He maybe has issues with his memory. It was actually a female president, not of a country, but of the European Union."
He was referring to European Commission President Ursula von der Leyen and a conversation she had with Trump in 2020.
"We cannot flip a coin about our security every four years depending on this or that election, namely the US presidential election," Mr Breton said.
He said EU leaders understood the bloc needed to boost its own military spending and capacities.
Nato's 31 members have agreed on a target of spending at least 2 per cent of gross domestic product on defence, but Nato estimates have shown that only 11 members are spending that much.