Donald Trump has said he would "encourage" Russia to attack any Nato member that fails to pay its bills as part of the Western military alliance,
BBC reports.
At a rally on Saturday, he said he had once told a leader he would not protect a nation behind on its payments, and would "encourage" the aggressors to "do whatever the hell they want".
Members of Nato commit to defend any nation in the bloc that gets attacked.
The White House called the comments "appalling and unhinged".
Nato chief Jens Stoltenberg said any suggestion that "allies will not defend each other undermines all of our security", putting soldiers from Nato countries at risk.
Addressing crowds during the rally in South Carolina, Mr Trump said he had made his comments about Russia during a meeting of leaders of Nato countries.
He recalled that the leader of a "big country" had presented a hypothetical situation in which he was not meeting his financial obligations within Nato and had come under attack from Moscow.
Mr Trump said the leader had asked if the US would come to his country's aid in that scenario, which prompted him to issue a rebuke.
"I said: 'You didn't pay? You're delinquent?'... 'No I would not protect you, in fact I would encourage them to do whatever they want. You gotta pay.'"
A White House spokesman said the former president was "encouraging invasions of our closest allies by murderous regimes" and labelled the comments "appalling and unhinged".
He added that the statement "endangers American national security, global stability and our economy at home".
In a statement responding to Mr Trump's comment on Sunday, Nato Secretary General Jens Stoltenberg said: "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."
