Biden says nuclear threat biggest since Cold War

US President Joe Biden said Russian President Vladimir Putin's threat to use nuclear weapons is the biggest such threat since the Cuban Missile Crisis, as Russia's military leadership faced a rare domestic public backlash over the war in Ukraine.

President Volodymyr Zelensky said Ukraine's forces were swiftly recapturing more territory especially in the south of the country as Mr Putin's seven-month invasion unravels.

Mr Biden said the United States was "trying to figure out" Mr Putin's off-ramp from the war, warning that the Russian leader was "not joking when he talks about potential use of tactical nuclear weapons or biological or chemical weapons, because his military is, you might say, significantly underperforming".

"For the first time since the Cuban Missile Crisis, we have a direct threat to the use of nuclear weapons, if in fact things continue down the path they'd been going," Mr Biden told Democratic donors in New York.

"We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis," he said.