Former United States Secretary of Defense Robert Gates on Saturday blasted Republican presidential nominee Donald Trump as being "unqualified and unfit to be commander-in-chief", the AFP news agency reported.
At the same time, Gates also issued a sharp critique of Trump's Democratic rival, Hillary Clinton.
Gates, a Republican who was Pentagon chief under both Republican President George W. Bush and Democrat President Barack Obama, has worked with eight presidents and is among the most widely respected American voices on national security matters.
"I believe Mr. Trump is beyond repair. He is stubbornly uninformed" and "temperamentally unsuited to lead our men and women in uniform," Gates was quoted as having written in an op-ed on the Wall Street Journal's website. "He is unqualified and unfit to be commander-in-chief."
At the same time Gates, who worked closely with Hillary Clinton when she was Secretary of State, did not hold back on his criticism of her tenure.
He said that Clinton's failure to predict the chaos that would follow Western intervention in Libya had raised credibility questions, as had her shifting position on the Iraq war.
Gates assailed Clinton's opposition to a pending Asian trade agreement she once supported, said she had been vague on dealing with Vladimir Putin's Russia, said she offered few specifics about North Korea, and had no clear strategy toward "a Middle East in flames."
But on credibility, Gates added, "Trump is in a league of his own."
"He has expressed support for building a wall between the US and Mexico; for torturing suspected terrorists and killing their families (and) for Mr. Putin’s dictatorial leadership."
Trump is "willfully ignorant" about the world, the U.S. military and "about government itself," Gates charged.
He did not say he would vote for Clinton, but did write that he would be listening for reassurances from her ahead of the November 8 election.
Despite Gates' scathing criticism of Trump, the Republican nominee recently earned the endorsement of 88 retired generals and admirals.
The Republican nominee's campaign unveiled an open letter in which the retired military leaders warned of the "potentially extremely perilous" combination of budget cuts and policy choices will "emboldened" enemies of the country as a result of those actions.