'BEYOND REPAIR': Former US Defense Secretary Robert Gates tears into Donald Trump

Former US Defense secretary, Robert Gates is having doubts about both Hillary Clinton and Donald Trump.

Gates, who has served eight US presidents including George W. Bush and Barack Obama over the course of 50 years, said there is still too much that we do not know about Clinton's and Trump's foreign-policy positions.

"Neither candidate has seriously addressed how he or she thinks about the military, the use of military force, the criteria they would apply before sending that force into battle, or broader questions of peace and war," Gates wrote in a Wall Street Journal column published on Friday.

Gates noted that both the Democratic and Republican presidential nominees have largely stuck to speaking in generalities about the complex issues that they would face after being sworn into office.

Those issues, Gates asserted, are numerous and fraught with potential trap doors. Among them, China's claims to the South China Sea, North Korea's nuclear ambitions, Russian President Vladimir Putin's authoritarian regime, Iran's aggression, and the long-playing unrest in the Middle East, which is compounded by ISIS-related terrorism and the Syrian civil war.

According to Gates, dealing with China in particular "requires a president with strategic acumen and vision, nuance, deft diplomatic and political skill." He suggested that such qualities are lost on a presidential candidate like Donald Trump.

Writing about the real-estate mogul's hawkish position on China, Gates said, "All we really know is Mr. Trump's intention to launch a trade war with a country holding over $1 trillion in U.S. debt."

He took a swipe at Hillary Clinton's opposition to the Trans-Pacific Partnership - a trade agreement she initially endorsed - "the failure of which would hand China an easy political and economic win," Gates wrote.

Gates saved his most searing rebukes for Trump, who has built the foreign-policy arm of his presidential campaign on bluster and innuendo - the likes of which have rattled US allies around the world.

While noting that both Clinton and Trump have credibility issues on foreign policy, Gates added that "Donald Trump is in a league of his own" in that regard.

Here's Gates:

"The world we confront is too perilous and too complex to have as president a man who believes he, and he alone, has all the answers and has no need to listen to anyone ... At least on national security, I believe Mr. Trump is beyond repair. He is stubbornly uninformed about the world and how to lead our country and government, and temperamentally unsuited to lead our men and women in uniform. He is unqualified and unfit to be commander-in-chief."