Hours after Iran shot down a US spy drone, sending tensions soaring, the former vice president said Trump was failing to prevent the Islamic republic from obtaining a nuclear weapon and to secure energy supplies through the Strait of Hormuz.

"President Trump's Iran strategy is a self-inflicted disaster," Biden, who leads in polling for his party's 2020 nomination, said in a statement.

"By walking away from diplomacy, Trump has made military conflict more likely."

Biden served nearly four years as chairman of the powerful Foreign Relations Committee during his 36 years in the Senate, and considers foreign policy a strong suit.

As vice president, he helped president Barack Obama negotiate the 2015 agreement between Iran and world powers that imposed curbs on its nuclear program, blocking its path to an atomic weapon.

But Trump unilaterally withdrew from the agreement last year, and as a result, Biden said, "Iran is building back up its nuclear capability."

While Trump promised that exiting the deal and slapping sanctions on Iran would brake its belligerence in the region, "they've only gotten more aggressive," Biden said.

"It's sadly ironic that the State Department is now calling on Iran to abide by the very deal the Trump Administration abandoned."

Biden stressed that Iran continues to be a "bad actor that abuses human rights" and supports terrorism. "But what we need is presidential leadership that will take strategic action to counter the Iranian threat" and improve cooperation with US allies.

With tensions soaring, Iran will likely be a topic when Biden and 19 other candidates in the crowded Democratic field gather in Miami over two nights next week for the first primary debate of the 2020 cycle.