Democratic presidential candidate Hillary Clinton said Thursday the United States is stronger and safer thanks to the "power of allies," warning that her Republican rival, Donald Trump, would significantly weaken the relationships.

In a foreign policy speech, Clinton cited as an example how the U.S., South Korea and Japan worked together to cope with threats from North Korea, a country she called "the most repressive regime on the planet run by a sadistic dictator who wants to develop long-range missiles that could carry a nuclear weapon to the United States."

"When I was secretary of state, we worked closely with our allies Japan and South Korea to respond to this threat, including by creating a missile defense system that stands ready to shoot down a North Korean warhead should its leaders ever be reckless enough to launch one at us," Clinton said.

She said the three countries plan to hold a joint drill to test the system later this month.

"That's the power of allies. It's also the legacy of American troops who fought and died to secure those bonds because they knew we were safer with friends and partners," she said in the speech in San Diego, California.

Russia and China are "deeply envious of our alliances around the world" as they have nothing like them, she said.

"They'd love for us to elect a president who would jeopardize that source of strength. If Donald gets his way, they will be celebrating in the Kremlin. We cannot let that happen," she said.

Clinton also emphasized how little value Trump attaches to alliances, saying Trump has suggested "America should withdraw our military support for Japan, encourage them to get nuclear weapons."

"He said this about a war between Japan and North Korea and I quote, 'If they do, they do. Good luck. Enjoy yourself, folks.' I wonder if he even realizes he's talking about nuclear war," she said.

Referring to Trump's calls for allies to pay up for U.S. protection, Clinton said, "Yes, our friends need to contribute their fair share. I made that point long before Donald Trump came on to the scene and a number of them have increased their defense spending."

"The real debate here is whether we keep those alliances strong or cut them off. What he says would weaken our country," she said.

Clinton also accused Trump of having "bizarre fascination with dictators and strong men who have no love for America."

"He praised China for the Tiananmen Square massacre. He said it showed strength. He said you've got to give Kim Jong-un credit for taking over North Korea, something he did by murdering everyone he saw as a threat, including his own uncle, which Donald described gleefully like he was recapping an action movie," she said. "And, he said if he were grading Vladimir Putin as a leader, he'd give him an A."

Clinton said Trump is "temperamentally unfit" to be president.

"This is not someone who should ever have the nuclear codes because it's not hard to imagine Donald Trump leading us into a war just because somebody got under his very thin skin," she said. (Yonhap)