Nato cannot be an 'a la carte' military alliance dependent on the whims of the US president, the European Union's foreign policy chief Josep Borrell has said.
Mr Borrell was responding to comments made by former US president Donald Trump regarding Nato over the weekend.
Speaking at a rally in South Carolina, Mr Trump suggested the US might not protect Nato allies which do not spend enough on their own defence from a potential Russian invasion.
Asked to respond to Mr Trump's comments, Mr Borrell said on Monday: "Nato cannot be an 'a la carte' military alliance... depending on the humour of the president of the US."
Following his remarks, Mr Trump was faced with criticism from some of his Republican Party colleagues.
"This is why I've been saying for a long time that he's unfit to be president of the United States," former Republican presidential candidate Chris Christie said in an interview with NBC.
During Saturday's political rally, Mr Trump complained about what he called "delinquent" payments by some Nato countries, and recounted what he said was a past conversation with the head of "a big country" about an attack by Russia on such countries.
'Appalling and unhinged'
"No, I would not protect you. In fact, I would encourage them (Russia) to do whatever the hell they want. You gotta pay," Mr Trump said he told the unnamed leader.
The remarks prompted rebukes from the White House, which called them "appalling and unhinged," as well as from other top Western officials.
The failure of many of Nato's 31 members to meet a defence spending target of at least 2 per cent of gross domestic product has long been a source of tension with the US, whose armed forces form the core the alliance's military power.
Nato estimates have shown that only 11 members are spending at the target level.
Former South Carolina governor Nikki Haley, who is Mr Trump's lone remaining challenger for the 2024 Republican presidential nomination, said: "The last thing we ever want to do is side with Russia."
Interviewed on CBS, she added: "Don’t take the side of someone who has gone and invaded a country and half a million people have died or been wounded because of (Russian president Vladimir) Putin."
Republican senator Lindsey Graham, a close ally of Mr Trump's, said he disagreed "with the way he (Trump) said it," referring to the Nato remarks.
Graham added: "Russia didn't invade anybody when he was president and if he's president again they won't."
Virtually every American president at some point, in some way, has complained about other countries in Nato not doing enough.
Politico reported that Republican Senator Thom Tillis blamed Trump aides for failing to explain to the former president that the US, as a Nato member, is committed to defending any member of the alliance that is attacked.
It also quoted Republican Senator Rand Paul saying Mr Trump's remarks were a "stupid thing to say".
On a day in which the US Senate was holding a rare weekend of debates over US president Joe Biden's request for emergency aid to help Ukraine repel a nearly two-year-old Russian attack and Israel in its war with Hamas in Gaza, some Republicans defended Mr Trump.
"Virtually every American president at some point, in some way, has complained about other countries in NATO not doing enough.
"Trump's just the first one to express it in these terms, but I have zero concern, because he's been president before," Senator Marco Rubio said in an interview with CNN.
In a statement, senior Trump campaign advisor Jason Miller sidestepped the remark encouraging Russia to take on some NATO allies, saying: "Democrat and media pearl-clutchers seem to have forgotten that we had four years of peace and prosperity under President Trump," while levelling attacks against Biden.
On Capitol Hill, Democratic Senator Peter Welch said Mr Trump "normalises belligerent behavior, but in fact he means it".
-Reuters
