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."

Russia has repeatedly warned that Ukraine could use a so-called dirty bomb on its own territory.

This is a conventional bomb laced with radioactive, biological or chemical materials which get disseminated in an explosion.

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, "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.