This was not the first time Mr. Biden has made off-the-cuff remarks about foreign policy during an evening fund-raiser. At another such event in New York in October, he warned donors that the world was closer to nuclear destruction than at any time since John F. Kennedy faced off against Nikita S. Khrushchev over Soviet missiles in Cuba in 1962. “We have not faced the prospect of Armageddon since Kennedy and the Cuban Missile Crisis,” Mr. Biden said then, prompting a nervous reaction.

In California on Tuesday, Mr. Biden was addressing Democratic contributors at the home of Mark Robinson, an investment banker, and his wife, Stephanie Robinson, who has worked as a management consultant and investment banker. In offering a wide-ranging review of international affairs, Mr. Biden focused on China, which has been on his mind lately. He used the occasion to try to tamp down American anxiety over Beijing’s economic rise and increasing assertiveness on the world stage.

“Don’t worry about China,” Mr. Biden said. “I mean, worry about China, but don’t worry about China,” he added to laughter. “No, but I really mean it. China is real — has real economic difficulties.”

After discussing the spy balloon, the president went on to say that Mr. Xi was frustrated by Mr. Biden’s efforts to bolster the Quad, an alignment of the United States, Japan, Australia and India seen as a counterpoint to Chinese influence in the Indo-Pacific region. Mr. Biden met with the Quad leaders last month on the sidelines of the Group of 7 meeting in Hiroshima, Japan.

“What he was really upset about was that I insisted that we reunite the so-called Quad,” Mr. Biden said. “He called me and told me not to do that because it was putting him in a bind. I said, ‘All we’re doing — we’re not trying to surround you, we’re just trying to make sure the international rules with air and sea lanes remain open.’ And we’re not going to yield to that — on that.”

“So now,” he added, “we have India, Australia, Japan and the United States working hand in glove in the South China Sea and the Indian Ocean.”

Julian E. Barnes contributed reporting.

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