US Libertarian Party presidential nominee Gary Johnson made an embarrassing gaffe on a morning talk show when he asked: "What is Aleppo?"

Questioned by MSNBC host Mike Barnicle on what he would do about Aleppo if elected president, Johnson seemed at a loss.

"About...?" he asked the host.

"Aleppo," Barnicle repeated.

"And what is Aleppo?" Johnson asked.

"You're kidding," Barnicle said, going on to tell Johnson that Aleppo is a city in Syria that is the "epicentre" of the refugee crisis.

"OK, got it, got it" Johnson said on hearing the explanation.

"Well with regard to Syria, I do think that it's a mess. I think the only way we deal with Syria is to join hands with Russia to diplomatically bring that to an end," he added on understanding what Aleppo was.

Johnson is currently polling numbers of around 9%, and is hoping to reach the 15% threshold to get into a presidential debate.

The libertarian candidate and former governor of New Mexico is openly critical of the US' two-party system that sees smaller or independent candidates largely excluded from popular politics.

He was previously a member of the Republican Party and some believe he has a chance of stealing a few votes from GOP members who aren't keen on backing Donald Trump.