More than 700 national security officials endorse Kamala Harris for president

The letter is a further boost to the vice-president and her bid for the White House. Since Joe Biden dropped his bid for re-election in July, Harris has opened up a narrow lead over Trump and performed more strongly in the crucial swing states needed for victory. She has also secured the endorsement of some key anti-Trump Republicans.

Among those signing the letter is the former secretary of state and 2016 presidential candidate Hillary Clinton, Jeff Bleich, who served as the US ambassador to Australia under Barack Obama, and the former CIA director John Deutch.

In her new book, Clinton expressed her excitement of the prospect of a woman becoming president.

Last month, Trump was pictured giving a thumbs up with family members at a ceremony to mark the deaths of US soldiers in Afghanistan. The army accused two campaign officials of pushing aside a worker at the cemetery who told them that it was not permitted to take photographs at the graves of recently deceased soldiers.