The Secretary General of NATO reacted harshly to Trump's words about the intimidation of allies

The Secretary General noted that NATO is both ready and able to defend all allies in the event of aggression, and any attack on a NATO country will receive a united and powerful response.

"Any suggestion that allies will not protect each other undermines all of our security, including that of the United States, and increases the risks to American and European soldiers. I expect that regardless of who wins (the presidential election), The US will remain a strong and loyal NATO ally," Stoltenberg said.

Donald Trump 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".

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".

Other Republicans chastised Mr Trump. The strongest criticism came from a surprising source: Rand Paul, the Kentucky senator known for his own isolationist leanings and skepticism of America’s military footprint.

“Stupid thing to say,” he told The Independent news outlet in the UK.

“I agree with Trump that they don't pay enough they should pay more, but saying that they should be invaded by Russia,” is not wise, Mr Paul continued.