Clinton says Trump dangerous, unfit to be commander in chief

Says Trump is unfit to hold an office that needs knowledge, stability, and immense responsibility

US presidential hopeful Hillary Clinton tore into her likely election rival Donald Trump as never before today, saying his foreign policy is dangerously incoherent and labeling him unfit for office.

"He is temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility," Clinton said, cranking up the rhetoric on what is already a deeply acrimonious election.

"This is not someone who should ever have the nuclear codes."

Americans will go to the polls on November 8 to elect a president to replace Barack Obama.

Clinton and Trump are all-but assured to be nominated as the Democratic and Republican candidates.

Offering a survey of the world's security challenges, the former secretary of state who is vying to become the first female president in America's 238-year history pilloried Trump for embracing tyrants and annoying allies, while trying to showcase her own grasp of world affairs.

"Even if I weren't in this race, I'd be doing everything I could to make sure Donald Trump never becomes president because I believe he will take our country down a truly dangerous path," she told an audience in San Diego, California.

Touching on his praise for Russian President Vladimir Putin and North Korean leader Kim Jong-un, Clinton ridiculed the former reality TV star, voicing incredulity at his views.

"I will leave it to the psychiatrists to explain his affection for tyrants," she said.

Clinton's fiery speech foreshadows what is certain to be one of the most vitriolic US elections in living memory.

As she was speaking, Trump tweeted: "Bad performance by Crooked Hillary Clinton! Reading poorly from the teleprompter! She doesn't even look presidential!"

With the two major parties deeply divided by rancorous primary campaigns, both leaders are likely to use a common enemy to unify supporters.