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.

“I spent a lot of time today talking about that,” Biden said in response to the reporter’s question. “Let me just say: Russia would be making an incredibly serious mistake for it to use a tactical nuclear weapon.

“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.

“The purpose of this heinous provocation is clear—to accuse Russia of using a weapon of mass destruction,” Zakharova said. “The Ukrainian authorities and their Western handlers hope that it will lead to a broad anti-Russian campaign, undermine Moscow’s credibility in the eyes of its partners and lead to our country’s isolation in the international arena.”

Western leaders have dismissed the allegation. In a joint statement released Sunday, the foreign ministers of the U.S., U.K. and France said that the defense ministers of their respective countries had spoken with Russian Defense Minister Sergei Shoigu at his request earlier that day.

“Our countries made clear that we all reject Russia’s transparently false allegations that Ukraine is preparing to use a dirty bomb on its own territory,” the statement read. “The world would see through any attempt to use this allegation as a pretext for escalation. We further reject any pretext for escalation by Russia.”

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.

Kremlin spokesperson Dmitry Peskov told Newsweek in a statement that “this nuclear rhetoric is not being [heated] up from [Russia’s] side.”

“On the contrary it is being done [quite] irresponsibly from several European capitals and the U.S.,” the statement added.

Updated 10/28/22, 2:30 p.m. ET: This story has been updated with a statement from the Kremlin.