The former US president had said he would let Russia "do whatever the hell they want" to members of the alliance that don't pay enough for defence.
NATO has criticised comments made by former US President Donald Trump at a campaign event - claiming they "put American and European soldiers at increased risk".
The former US president had said he would let Russia "do whatever the hell they want" to members of the alliance that don't pay enough for defence.
At a rally in South Carolina on Saturday, Mr Trump recalled an exchange with an unnamed "big country" who asked if they would be protected if Russia attacked them.
"I said: 'You didn't pay? You're delinquent?' No, I would not protect you. In fact, I would encourage them to do whatever the hell they want. You got to pay. You got to pay your bills," he told the crowd.
Sky News reports that in response, NATO said: "NATO remains ready and able to defend all allies. Any attack on NATO will be met with a united and forceful response.
"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.
"I expect that regardless of who wins the presidential election the US will remain a strong and committed NATO ally."
Mr Trump's comments will likely increase anxiety in Europe about the US withdrawing military support from Ukraine should he be re-elected later this year.
The former president has also long criticised NATO, threatening to pull the US from the alliance, and butting heads with the leaders of other member states.
One of his campaign commitments is to "fundamentally" reevaluate "NATO's purpose and mission".
In a bid to reassure fellow NATO members, President Joe Biden signed a £685m ($886m) defence bill in January that seeks to block any president from unilaterally withdrawing from NATO.
The White House described Trump's comments as "appalling and unhinged".
"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," White House spokesperson Andrew Bates said in a statement Saturday night.
"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."