NATO chief Jens Stoltenberg has warned against talk that "undermines security" after former US president Donald Trump threatened to encourage Russia to attack NATO members who were not fully paid up.
"Any suggestion that allies will not defend each other undermines all of our security, including that of the US," Stoltenberg said.
Mr Stoltenberg's comments were echoed by European Council President Charles Michel, who called Mr Trump's comments "reckless".
"The Transatlantic Alliance has underpinned the security and the prosperity of Americans, Canadians and Europeans for 75 years," said Mr Michel on X, formerly Twitter.
"Reckless statements on #NATO's security and (Article) 5 solidarity serve only Putin's interest" and "they do not bring more security or peace to the world", he added.
"On the contrary, they re-emphasise the need for the #EU to urgently further develop its strategic autonomy and invest in its defence," he continued.
US President Joe Biden slammed the comments as "appalling and dangerous", warning that Mr Trump intends to give Russian leader Vladimir Putin "a greenlight for more war and violence".
"Donald Trump's admission that he intends to give Putin a greenlight 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 in a statement
Article 5 stipulates that if a NATO country is victim of an armed attack, each member of the Alliance must consider the act as an attack against the whole group and take measures to provide aid to the country under threat.
Mr Trump earlier threatened, in the event he is re-elected in the United States, that he would not defend NATO members who had not met their financial obligations, and would "encourage" Russia to attack them.
The former US president has regularly criticised allies of the North Atlantic Treaty Organization for underfunding the group.
During a meeting in South Carolina, Mr Trump said he had a conversation with the head of state of a NATO member, without naming them.
This statement came after Mr Trump, likely the candidate to face Democrat President Joe Biden in November's election, put pressure on Republicans in Congress to block a bill to provide new aid to Ukraine.
Trump comments on Russia, NATO 'appalling and unhinged' - White House
The White House rejected Mr Trump's comments as "appalling and unhinged."
Mr Trump, appearing to recount a meeting with NATO leaders during a political rally quoted the president of "a big country" that he did not name 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."
White House spokesperson Andrew Bates, asked about Mr Trump's comments, said: "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."
Mr Trump, frontrunner for the Republican presidential nomination, was a fierce critic of the NATO when he was president, repeatedly threatening to pull out of the alliance. He cut defence funding to NATO and frequently complained that the United States was paying more than its fair share.
Mr Bates said President Joe Biden, a Democrat seeking re-election in the November election, had restored US alliances after taking office in 2021, ensuring that NATO was now "the largest and most vital it has ever been."
"Rather than calling for wars and promoting deranged chaos, President Biden will continue to bolster American leadership and stand up for our national security interests, not against them," he said in a statement.
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With Mr Trump leading the US president in some polls, European allies worry a Trump victory in November could jeopardise the US commitment to the alliance, but NATO Secretary General Jens Stoltenberg last month said he did not think a second Trump Presidency would jeopardise US membership.
Mr Stoltenberg, who has been pushing member states to boost defence spending, said European allies were increasing their military contributions and "moving in the right direction."
Mr Trump has continued to hammer the transatlantic alliance, telling a campaign rally last month that he did not believe NATO countries would support the United States if it were attacked.
On Russia's war in Ukraine, Mr Trump has called for de-escalation and complained about the billions spent so far, although he has put forward few tangible policy proposals.
Since Russia's full-scale invasion in February 2022, US aid to Ukraine has totalled around $75 billion, Mr Stoltenberg said, while other NATO members and partner states combined have provided more than $100 billion.
Trump cheers border bill collapse, vows 'deportation operation'
Meanwhile, Mr Trump celebrated the collapse of legislation targeting the migrant crisis on the US-Mexico border, while vowing that, if re-elected, he would carry out a massive "deportation operation" on his first day in office.
The death of the bipartisan bill in the US Senate this week highlights the ex-president's iron grip on the Republican Party, as politicians acceded to his desire to deny US President Joe Biden a win on immigration.
"Let's not forget that this week we also had another massive victory that every conservative should celebrate. We crushed crooked Joe Biden's disastrous open borders bill," Mr Trump declared at a rally in South Carolina.
Under pressure from Mr Trump, who wants to exploit Mr Biden's perceived weakness on immigration, republican politicians appeared to decide that they would prefer stopping any border reforms until after November's election.
Mr Trump declared that deporting migrants would be one of his first tasks.
"On day one I will terminate every open border policy of the Biden administration and we will begin the largest domestic deportation operation in American history. We have no choice."
The Senate border bill had included aid for Ukraine and Israel, with lifelines for the US allies appearing dead in the water after republicans rejected it Wednesday.
The upper chamber is now considering a foreign aid package that decouples the aid from the border issue entirely.
The $95 billion (€88.07 billion) package set to be debated next week includes funding for Israel's fight against Hamas militants and for key strategic ally, Taiwan.
The lion's share, however, would help pro-Western Ukraine restock depleted ammunition supplies, weapons and other crucial needs as it enters a third year of war with Russia.
At the rally in South Carolina, the former president also criticised Nikki Haley, his former UN ambassador who is also seeking the Republican Party's nomination.
Ms Haley's bid is almost certainly doomed as she trails behind her ex-boss in the race by a large margin.
Addressing voters in Ms Haley's home state, Mr Trump questioned the whereabouts of her husband Michael, who has not been seen on the campaign trail as he is on a yearlong military deployment to the Horn of Africa country of Djibouti.
"Where's her husband? Oh, he's away. He's away. What happened to her husband? What happened to her husband," he said, raising his voice for dramatic effect.
"Where is he? He's gone. He knew. He knew."
Ms Haley later responded on social media platform X.
"Michael is deployed serving our country, something you know nothing about. Someone who continually disrespects the sacrifices of military families has no business being commander in chief," she said.
