Democratic presidential hopeful Hillary Clinton assailed Republican candidate Donald Trump on Thursday as "not just unprepared" to be president but "temperamentally unfit" to hold the office.

Clinton, who served as secretary of state under President Barack Obama, blasted Trump for failing to outline a coherent foreign policy doctrine and for his lack of knowledge about international affairs.

Clinton charged that Trump could not be trusted to lead the US military or handle its arsenal of nuclear weapons and said electing Trump would risk rolling back decades of foreign policy under both Democratic and Republican presidents.

"I believe the person the Republicans nominated for president cannot do the job," she said in remarks in San Diego, California.

Trump's call to reevaluate the US commitment to the NATO alliance, for the proliferation of nuclear weapons and for a resumption of the use of torture by the US demonstrate a lack of knowledge about the United States and the world, Clinton said.

Clinton stressed that the US needs to "stick with our allies" in the face of calls by Trump to reconsider the role of the US on the world stage.

Russia and China lack the strength of US alliances and would love to see them weakened under a potential Trump presidency, Clinton said.

"If Donald Trump wins, they'll be celebrating in the Kremlin. We cannot let that happen," she said.

She charged that Trump did not understand how to deal with sometime rivals such as Russia and China because he "doesn't see the complexity" that allows the US to cooperate with Moscow and Beijing on some issues, while disagreeing on others.

She also lambasted Trump remarks praising dictators such as North Korea's Kim Jong Un and his lack of concrete plans to confront terrorist groups like Islamic State.

It is unclear how Trump would confront Islamic State, but a Trump presidency would embolden the group by giving it recruitment fodder with his pledge to bar all Muslims from entering the US, Clinton said.

"This isn't reality television, this is actual reality," she said of Trump, who once starred in his own reality show, The Apprentice.

Trump, who began his campaign last year by vowing to build a wall along the US-Mexican border to keep out illegal immigrants, would also exacerbate relations with the United States' neighbours.

"We're lucky to have two friendly neighbours on our land borders. Why would he want to make one of them an enemy?" Clinton said.

Trump took to Twitter, his preferred medium for lambasting critics, to denounce Clinton for a "bad performance" and charge her with a lack of credibility.

In his own foreign policy address in April, Trump provided few specifics but said the US has lacked a coherent foreign policy since the end of the Cold War and that "America first" would be the overriding theme of his administration.

Clinton was not the only Democrat to challenge Trump on foreign policy on Thursday. Obama told graduates at the US Air Force Academy earlier that the US should not retreat from the world stage.

"America cannot shirk the mantle of leadership," Obama said during a graduation speech at the US Air Force Academy in Colorado. "We can't be isolationist, it's not possible in this global, interconnected world."

Ahead of Clinton's speech, Republicans pointed to what they say are Clinton's failures as the top US diplomat, including what they charge is her reluctance to say the US is at war with Islamic State and that military intervention in Libya created a power vacuum for radical Islamists.

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