Cringe-inducing moment US presidential hopeful Gary Johnson makes shock confession that he’s never heard of war-ravaged Syrian city

LONG-SHOT US presidential hopeful Gary Johnson hasn't done his campaign any favours -- after asking "what is Aleppo?" during a live TV interview.

The right-wing challenger for the White House made the embarrassing gaffe as he was being grilled over his foreign policy plans.

Seasoned political pundit Mike Barnicle asked him live on channel MSNBC: "What would you do, if you were elected, about Aleppo?"

But the straightforward question about the war-torn Syrian city at the centre of the country's civil war seemed to fall on deaf ears.

Dumbfounded Johnson replied: "And what is Aleppo?"

Libertarian Johnson -- who despite being a former Republican governor is running against Donald Trump in the US presidential elections -- then assures his host that he's "not kidding".

Barnicle explains: "Aleppo is in Syria. It is the epicentre of the refugee crisis".

His guest then replies "OK, got it, got it", before delivering a rambling analysis of the Syrian conflict.

He said the US should "join hands" with Russia to sort out what he called "the mess" in the country.

Co-host Joe Scarborough later gave a grave assessment of Johnson's capabilities.

"Boy, that’s depressing," he said after the interview.

"It is staggering that somebody would run for President of the United States, get 14 per cent, 15 per cent in polls, and be so ignorant on foreign policy they would ask the question 'what is Aleppo?' on national television."

On Wednesday, former Republican presidential candidate and firm critic of Donald Trump said he hoped Johnson would get a fair hearing during the campaign.

He tweeted: "'I hope voters get to see former GOP Governors Gary Johnson and Bill Weld on the debate stages this fall".

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