WASHINGTON -- US Democratic presidential nominee Joe Biden said Thursday that his election would mean that the United States will no longer cozy up to dictators, reiterating his commitment to strengthening alliances as he accepted the nomination.

"I'll be a president who stands with our allies and friends, and make it clear to our adversaries that days of cozying up to dictators is over," the former US vice president said in his nomination acceptance speech at the end of the Democratic Party's four-day national convention.

Biden was formally nominated Tuesday over his Democratic rival Bernie Sanders.

The 77-year-old said he, if elected, will work to build a more secure and more peaceful world.

"I will always stand for our values of human rights and dignity. I'll work in common purpose for a more secure, peaceful and prosperous world," he told the virtual national convention.

He said he would very personally take the responsibility of serving as commander in chief, noting his eldest son, Beau who died in 2015 of cancer, had served in the military.

"Under President Biden, America will not turn a blind eye to Russian bounties on the heads of American soldiers. Nor will I put up with foreign interference in our most sacred democratic exercise, voting," added Biden in a firm, determined voice.

The Democratic Party has long accused Trump of embracing dictators or "thugs," including North Korean leader Kim Jong-un.

Trump, however, insists his relationship with Kim was what had prevented what he claimed to be an imminent war under former President Barack Obama.

"It would have been a war if you had Hillary Clinton. It would have been a war if Obama were allowed to stay any longer," he said earlier Thursday while visiting Pennsylvania, the home state of Biden.

"President Obama said when I sat down (in) that first meeting ... he said it was the biggest problem we had, North Korea. And we would have had a big problem, would have been a hell of a war. Tell you right now. You'd probably be in that war right now," Trump said.

Trump has held three meetings with the North Korean leader, including the first-ever US-North Korea summit held in June 2018 that was followed by their second summit in Vietnam in February 2019.

Later in a telephone interview with Fox News, the US president claimed Biden may not be mentally or physically fit to deal with leaders, such as Kim and Chinese President Xi Jinping, whom he said were at the "top of their games."

"You are dealing with all of these people. Kim Jong-un, of course ... I can tell you one thing. They are very sharp," Trump said. "They are at the top of their games and if you are not at the top of your game, it's not going to be a pretty picture." (Yonhap)