The US presidential campaign's third candidate Gary Johnson has made another on-air foreign policy gaffe, failing to name a single foreign leader he admires and then proclaiming to be having another "Aleppo moment".

Johnson, who has been presented by his staff as a viable, libertarian alternative to Hillary Clinton and Donald Trump, was referring to a previous interview in which when asked what his solution to the ongoing crisis in Aleppo would be he replied: "What is Aleppo?"

Asked on Wednesday (28 September) in an interview in front of a live audience by MSNBC host Chris Matthews who his favourite world leader was, Johnson gave his now familiar vacant stare before trying former president of Mexico Vicente Fox. Fox was voted out of office in 2006.

Both Matthews and Johnson's co-interviewee, his running mate former Massachusetts Governor William F Weld, stalled to give the candidate more time. "Any one of the continents, any country, name one foreign leader that you respect and look up to, anybody," the MSNBC journalist said.

"Mine was Shimon Peres," Weld said. The Nobel Laureate Peres left office in Israel in 2014 and recently died.

"I'm giving you the whole world!" Matthews said. "Any one of the continents, any country, name one foreign leader that you respect and look up to, anybody," he added.

The painful exchange lasted for longer than 50 seconds but eventually concluded when Weld finally rescued Johnson by saying German chancellor, Angela Merkel. The answer was met with applause.

Earlier in September when Johnson was asked by MSNBC host Mike Barnicle what he would do about Aleppo if elected president, he 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.