US senator and former presidential candidate John McCain has claimed Kentucky senator Rand Paul is “now working for Vladimir Putin” in a row over Montenegro’s bid for Nato membership.

While speaking in the Senate house in support of a bill to advance Montenegro’s application to join the military alliance, Mr McCain introduced the bill by saying any opposition to it was in support of Russia’s aims.

He said: “If there is objection, you are achieving the objectives of Vladimir Putin. You are achieving the objectives of trying to dismember this small country which has already been the subject of an attempted coup.”

In remarks directed at Mr Paul, Mr McCain added: “I’ve no idea why anyone would object to this, except that I will say, if they object, they are now carrying out the desires and ambitions of Vladimir Putin, and I do not say that lightly.”

After presenting the bill, Mr Paul then stood up and said: “I object,” before immediately walking out of the chamber.

Mr McCain responded to the dramatic exit by re-iterating his remarks about Mr Paul working for Mr Putin.

He said: “The only conclusion you can draw when he walks away is that he has no argument to make, he has no justification for his objection to having a small nation be part of Nato, it is under assault from the Russians.

“So I repeat again, the senator from Kentucky is now working for Vladimir Putin.”

In a later statement, his office added: “Senator McCain, and certainly the people of Montenegro, would appreciate an explanation from Senator Paul as to why he sought to prevent this small, brave country from joining in the defence of the free world.”

In response, Mr Paul later said his colleague’s accusations in the Senate were “over the top”, and said McCain had become “a little bit unhinged”.

“You know, I think he makes a really, really strong case for term limits. I think maybe he’s past his prime”, Mr Paul told MSNBC.

Mr Paul also told The Daily Beast he believed the US should not support the expansion of Nato given the country’s debt. He said: “Currently, the United States has troops in dozens of countries and is actively fighting in Iraq, Syria, Libya, and Yemen (with the occasional drone strike in Pakistan).”

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