Biden has warned that a Russian nuclear strike would be a ‘huge mistake’.

Joe Biden warned on Tuesday that Russia’s use of nuclear weapons would be a ‘huge mistake’, as Moscow says Ukraine is building a ‘dirty bomb’, a move denied by Kyiv and the West.

• Read more: “Dirty Bomb”: An accusation tactic already used by Moscow

• Read more: “No one will be fooled” by a “dirty bomb” in Ukraine

• Read more: Ukraine accuses Moscow of planning mass deportations of residents of occupied territories

“If Russia were to use a tactical nuclear weapon it would be a huge mistake,” the US president said.

A radioactive bomb or “dirty bomb” consists of conventional explosives surrounded by radioactive material that is dispersed as dust during detonation.

The Pentagon warned Russia of the consequences of using such bombs. “If Russia uses nuclear weapons or a dirty bomb, there will be consequences,” Pentagon spokesman Gen. Pat Ryder told reporters.

Moscow made the allegations for the first time in phone conversations between Russian Defense Minister Sergei Shoigu and his American, French, British and Turkish counterparts on Sunday, referring to possible provocations by Ukraine with a “dirty bomb”.

Paris, London and Washington slammed Moscow’s “false” statements on Monday. “No one will be fooled by the attempt to use this as an excuse to escalate the charges,” they stressed in a joint statement.

“I’m not guaranteeing right now that this is a pretextual move. We just don’t know,” Joe Biden said Tuesday.