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 said this week that Ukraine could use a so-called dirty bomb on its own territory.

A dirty bomb 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 Tuesday, "I'm not guaranteeing that it's a false flag operation yet. We don't know."