US President Joe Biden warned Russia on Tuesday against using a nuclear weapon in the war with Ukraine, saying such an escalation would be a grave error, AFP reports.

Asked by reporters whether he thinks Russia is preparing a "dirty bomb" attack that it would then blame on Ukraine, Biden said, "Russia would be making an incredibly serious mistake were it to use a tactical nuclear weapon."

Earlier this week, Russian Defense Minister Sergei Shoigu alleged that Ukraine was preparing a “provocation” involving a dirty bomb, a device that uses explosives to scatter radioactive waste.

The US and its allies suspect Russia might itself use a dirty bomb in a "false flag" attack, possibly to justify use of conventional nuclear weapons by Moscow as it finds itself on the back foot in eastern and southern Ukraine.

Biden said on Tuesday, "I'm not guaranteeing that it's a false flag operation yet. We don't know."

The Pentagon said Tuesday that Russia would face repercussions for using such a device.

"If Russia were to employ nuclear weapons or a dirty bomb, there would be consequences," spokesman Brigadier General Pat Ryder told journalists, without providing specifics.

Shoigu’s accusations this week came amid global concerns that Russia may use a nuclear weapon against Ukraine.

Russian President Vladimir Putin issued a thinly veiled nuclear threat in a recent speech in which Putin also announced his country's first wartime military mobilization since World War Two.

Biden, in his speech at the United Nations General Assembly, accused Putin of making "overt nuclear threats against Europe" in reckless disregard for nuclear nonproliferation responsibilities.

US National Security Adviser Jake Sullivan later said that Washington takes Putin's nuclear threats seriously but does not presently see indications of the use of nuclear weapons by Russia.