Iran has been reduced to its weakest point in decades, President Joe Biden said in a valedictory foreign policy speech on Monday, citing Tehran's dire economy, knocked-out air defenses and loss of Syria as an ally.

"Iran's air defenses are in shambles. Their main proxy, Hezbollah, is badly wounded, and as we tested Iran's willingness to revive the nuclear deal, we kept the pressure with sanctions. Now Iran's economy is in desperate straits," Biden said in a White House speech to applause.

"All told, Iran is weaker than it's been in decades," he added, also citing Washington's and other allies' assistance of Israel in shooting down drones and ballistic missiles the Islamic Republic launched in two direct attacks last year.

Biden has been repeatedly criticized by President-elect Trump, who is due to return to the White House next week, as being too soft on Iran.

"You want more evidence we seriously weakened Iran and Russia? Let's take a look at Syria. President Assad was both countries' closest ally in the Middle East. Neither could keep him in power. Quite frankly, neither really tried very hard," Biden added.

"Now I cannot claim credit for every factor that led to Iran and Russia growing weaker in the past four years," Biden added. "They did plenty of damage all by themselves, (and) Israel did plenty of damage to Iran and its proxies, but there's no question our actions contributed significantly," he said without elaborating.

Hawkish Israeli Prime Minister Benjamin Netanyahu has said not heeding US warnings to ease up attacks on Iran's armed allies help put them on the backfoot.

"Now major authoritarian states are aligning more closely - Iran, Russia, China, North Korea - but that's more out of weakness than out of strength," Biden said.