Trump: We have nuclear weapons, why not use them?

The row came amid speculation that Mr Trump's campaign was in turmoil and unravelling.

Republican presidential candidate Donald Trump. (AP)

Donald Trump asked a foreign policy expert three times why the US couldn't use nuclear weapons if he becomes president, it has been claimed. Mr Trump, the Republican nominee, was said to have posed the question during an hour-long briefing on foreign affairs, saying: "If we have nuclear weapons, why can't we use them?"

The claim was made by Joe Scarborough, a former Republican congressman and political talk show host on US television.

Hope Hicks, Mr Trump's aide, denied that such a conversation about using nuclear weapons ever took place. She said: "There is no truth to this."

Mr Scarborough was a friend of Mr Trump for a decade, enthusiastically supported the billionaire and was mentioned as a possible running mate.

The two fell out earlier this year, with Mr Scarborough saying the mogul was "not the man I once knew".

He claimed the nuclear conversation took place several months ago, but he had only been informed of it by the foreign policy expert in recent days.

The allegation emerged as Mr Scarborough was interviewing Michael Hayden, CIA director under President George W Bush. He asked Mr Hayden if any of his peers were advising Mr Trump on national security and foreign policy. Mr Hayden replied: "None."

Mr Scarborough then said: "I have to follow up on that... and I'll be very careful here. Several months ago a foreign policy expert, on an international level, went to advise Donald Trump and three times he [Mr Trump] asked about the use of nuclear weapons.

"He asked three times in an hour briefing, 'Why can't we use nuclear weapons?'."

The former CIA Director said he was "very concerned" by how "erratic" Mr Trump seemed.

Mr Trump has previously suggested that countries including Japan, South Korea and Saudi Arabia should have their own nuclear arsenal.

He also refused to rule out deploying weapons against Islamic State of Iraq and the Levant and in Europe.

In March Mr Trump was asked where he got military advice and replied: "From the (television) shows."

He subsequently revealed a short list of advisers that left some senior Republicans unimpressed. It was not clear whether one of those advisers was involved in the alleged conversation about using nuclear weapons.

The row came amid speculation that Mr Trump's campaign was in turmoil and unravelling.

In recent days he caused outrage by feuding with the parents of Captain Humayun Khan, a Muslim US soldier who died serving in Iraq. He also infuriated his own party by failing to back the re-election bids of Speaker Paul Ryan and Senator John McCain.