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.

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", the BBC wrote. 

The White House called the comments "appalling and unhinged."

Addressing crowds during the rally in South Carolina, 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.

Trump said the leader had asked if the U.S. would come to his country's aid in that scenario, which prompted him to issue a rebuke.