Former US defense secretary Robert Gates issued a sharp critique of both presidential candidates in the Wall Street Journal, but blasted Donald Trump as being "unqualified and unfit to be commander-in-chief."

Gates - a Republican who was Pentagon chief under Republican George W. Bush and Democrat Barack Obama - has worked with eight presidents and is among the most widely respected US 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 wrote in an op-ed piece posted late Friday on the Journal's website. "He is unqualified and unfit to be commander-in-chief."

Gates worked closely with Hillary Clinton when she was secretary of state, and did not hold back on his criticism of her tenure.

A former CIA director, Gates said 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 had expressed support "for withdrawing US troops from Europe, South Korea and Japan," while being "cavalier about the use of nuclear weapons." He had insulted servicemen, their families and military leaders, Gates wrote.

Trump is "willfully ignorant" about the world, the US military and "about government itself," Gates said.

While not saying he would vote for Clinton, Gates said he would be listening for reassurances from her ahead of the November 8 election.