US President, Joe Biden, has warned that Russia would be making a “serious mistake” if it uses a tactical nuclear weapon in its war with Ukraine.

The president was responding to a question from a reporter who asked about Russia’s allegation that Ukraine was planning to deploy nuclear weapons or a “dirty bomb” on its own territory. Some Western officials have warned that Russia’s accusation could be a pretext for Russian President Vladimir Putin’s military to escalate the war against Ukraine.

“Let me just say: Russia would be making an incredibly serious mistake for it to use a tactical nuclear weapon,”   Biden said in response to the reporter’s question.

“I’m not guaranteeing you that it’s a false-flag operation yet, don’t know. But it would be a serious, serious mistake,” the president said.

Russian state-owned news agency RIA Novosti said in a Telegram post Sunday that Ukraine was planning to use a dirty bomb on its own territory and then place the blame on Russia. A statement from Russian Foreign Ministry spokeswoman Maria Zakharova on Monday attributed Russia’s knowledge of Ukraine’s alleged dirty bomb plans to “information from reliable sources,” but it did not specify who or what those sources are.

A dirty bomb is a mix of explosives, such as dynamite, and radioactive powder or pellets, according to the Centers for Disease Control and Prevention (CDC). It cannot create an atomic blast, but an explosion from a dirty bomb can carry radioactive material into the surrounding area.

A dirty bomb blast can cause injuries and property damage. Though only people very close to the explosion site can be exposed to enough radiation that would cause “immediate serious illness,” radioactive dust and smoke can spread farther away and pose a danger to people who breathe it in or ingest contaminated food or water, the CDC said.

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