Donald Trump says he would "encourage" Russia to attack any Nato member that fails to pay its bills as part of the Western military alliance.
Axar.az reports that he said he had once told a Nato leader he would not protect a nation behind on its payments if it came under attack from Russia, and would "encourage them 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".
Speaking during a rally in South Carolina on Saturday, 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.