In a searing indictment of her Republican rival, Democratic presidential front-runner Hillary Clinton said Donald Trump is "unfit" for the White House and holds foreign policy ideas that are "dangerously incoherent."

Billed as a major foreign policy address, Clinton devoted much of the June 2 speech to attacking Trump while seeking to bolster her case that she would offer the country a steady hand in dealing with the rest of the word.

She said it would be a "historic mistake" to elect Trump, a wealthy businessman and reality TV star who has never worked in government and whom she accused of being too volatile and impulsive for the job.

"This is not someone who should ever have the nuclear codes," Clinton told an audience that included military members and their families in San Diego, California.

She suggested that Trump could lead the country into war "just because somebody got under his very thin skin."

Citing her experience as secretary of state, a U.S. senator, and first lady, Clinton said she has a record of "helping shape" U.S. foreign policy.

She described Trump’s foreign policy views as "not really even ideas" but rather a "series of bizarre rants, personal feuds, and outright lies."

Clinton challenged calls by Trump to reexamine the role of the United States in the world, saying Washington needs to "stick with our allies."

Trump has said that if other NATO members do not step up their spending on the military alliance, then Washington would "let countries defend themselves" if he is elected president.

Clinton accused Trump of threatening to "abandon our allies in NATO" at a time when "Moscow has taken aggressive military action in Ukraine," a reference to Russia’s illegal annexation of Ukraine’s Crimean Peninsula in March 2014 and backing of armed separatists in the country's east.

Clinton said Trump would be incapable of defending U.S. interests in dealing with countries like China and Russia, which she said "often work against" the United States.

Trump has vowed to seek better relations with Russia if he is elected, but only if Washington approaches the relationship "from a position of strength."

"Moscow and Beijing are deeply envious of our alliances around the world, because they have nothing to match them," Clinton said. "They’d love for us to elect a president who would jeopardize that source of strength."

"If Donald gets his way, they’ll be celebrating in the Kremlin," she added.

Trump has cited his interactions with foreign states in real-estate deals and beauty pageants as relevant to his foreign policy credentials, a claim that Clinton ridiculed.

"He says he has foreign policy experience because he ran the Miss Universe pageant in Russia," Clinton said, eliciting laughter from the crowd.

Clinton also defended last year’s nuclear agreement between Iran and world powers, under which Tehran curbed its nuclear program in exchange for the removal of economic sanctions.

Trump has said that his experience negotiating business deals would have allowed him to strike a better accord, and that he would have walked away from a bad agreement.

But Clinton said that while that might work in negotiating a golf course deal, "it doesn't work like that in world affairs."

On the Islamic State, Clinton said a Trump presidency would embolden the extremist group by giving it fodder for recruiting new militants with his pledge to bar all Muslims from entering the United States temporarily.

Trump criticized the speech on Twitter on June 2 without directly addressing Clinton’s accusations that he is unfit for the presidency.

He wrote that Clinton "no longer has credibility -- too much failure in office" and that "she doesn’t even look presidential."

"People will not allow another four years of incompetence," Trump wrote.

Trump later described Clinton's speech as a "hit job" at a rally in San Jose, California. At the same time, he said the speech was so boring it was "hard to stay awake" and joked that Clinton would make a lot of money delivering speeches to insomniacs.

The location of Clinton's speech was strategic. San Diego is home to several military installations, a large population of veterans and active duty personnel, and military-related industries.

The address also came five days before California holds its primary election, when she is hoping to defeat her tenacious Democratic rival, Senator Bernie Sanders, and clinch her party's nomination.

With reporting by dpa, Reuters, and AP