US President Joe Biden and his staff conveyed to his successor Donald Trump in a White House meeting on Wednesday that Iran presents the most immediate national security threat to the United States, the national security advisor said.

China presents the biggest possible challenge to the global standing of the United States, Jake Sullivan told reporters, but Iran poses a more direct threat.

"Then there's the most immediate issue, which is Iran and its proxy groups continue to take actions that directly threaten Americans and American interests in the Middle East, and that has to be dealt with on an urgent basis," Sullivan said.

Trump and Biden met in the Oval Office for around two hours a week after the former President trounced Vice President Kamala Harris in a decisive re-election which will give him broad leeway to reshape foreign and domestic policy.

Biden had attempted early in his tenure to revive an international deal on Iran's nuclear program from which Trump had withdrawn, saying the Obama-era agreement allowed Iran to shore up its finances and step up aid to armed allies in the Mideast.

In the intervening years, the Islamic Republic boosted revenues from oil sales, approached military grade uranium enrichment, aided Russia’s war in Ukraine and back allied armed groups in an ongoing region-wide fight against Israel.

Sullivan said he had disagreements with his incoming successor, former congressman and avowed Iran hawk Mike Waltz, but that he was prepared to work together for a smooth transition of power.

"He and I obviously don't see eye to eye on every issue, but I am very much looking forward to engaging him over these next 60 days, as I said, so that we can have this smooth handoff," Sullivan said.

The Biden administration would work toward the release of Israeli hostages and a ceasefire to wars in Gaza and Lebanon in the days that remain, Sullivan added.