NATO chief Jens Stoltenberg said any attack on the military alliance would be countered with "a united and forceful response," after US presidential candidate Donald Trump invited Russia to do "whatever the hell they want" to NATO members not meeting their financial commitments.
"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," Stoltenberg said in a statement on Sunday.
"I expect that regardless of who wins the presidential election the US will remain a strong and committed NATO ally."
Trump, the presumptive Republican nominee for the November election, said at a campaign event in the state of South Carolina on Saturday that he would "encourage" Russia "to do whatever the hell they want" to NATO countries that are not spending enough on defence.
"If we don't pay and we're attacked by Russia, will you protect us?," Trump said a leader of a "big country" had asked him while he was president.
Trump said he responded: "No, I would not protect you. In fact, I would encourage them to do whatever the hell they want. You got to pay."
The comment caused a stir in Europe, where memories are fresh of Trump's scepticism toward the trans-Atlantic alliance. He created a minor furore in 2017 when he called NATO "obsolete," and he frequently charged during his presidency that the US was paying an unfair share as a member.
"Reckless statements on NATO’s security and Article 5 solidarity serve only Putin’s interest. They do not bring more security or peace to the world," European Council President Charles Michel said on X, formerly Twitter.
A number of high-level politicians reacted in Poland, which shares a border with Russia.
"NATO's motto 'one for all, all for one' is a concrete commitment," wrote Defence Minister Władysław Kosiniak-Kamysz on X.
"Undermining the credibility of allied countries means weakening the entire North Atlantic Treaty Organization. No election campaign is an excuse for playing with the security of the alliance."
Poland is considered one of the closest US allies in Europe and is currently investing an above-average amount in its own armaments, but apparently still felt unsettled by Trump's comments.
Interior Minister Marcin Kierwiński was quoted by the news agency PAP as saying: "Trump is directly calling for Europe to be handed over to [Russian President Vladimir] Putin."
President Andrzej Duda, a right-wing populist who has expressed admiration for Trump, seemed to downplay the remark, saying that "the alliance between Poland and the USA must be strong, regardless of who is currently in power in Poland or the USA."
US President Joe Biden's White House condemned Trump immediately after the comments were made at the event in Conway, South Carolina.
"Encouraging invasions of our closest allies by murderous regimes is appalling and unhinged – and it endangers American national security, global stability, and our economy at home," said spokesman Andrew Bates said in a statement.
Biden has "restored our alliances," Bates said, adding that NATO is the "largest and most vital it has ever been."
Article 5 of the NATO treaty enshrines the promise of collective defence. It has been invoked only once, in the wake of the September 11, 2001 attacks by al-Qaeda on the United States.
In 2006, NATO defence ministers agreed as an informal target to commit a minimum of 2% of their country's gross domestic product to defence spending. Many NATO countries, including Germany, have yet to achieve that level.
