President Biden on Tuesday warned Russia against using a tactical nuclear weapon amid concerns Moscow may be creating a pretext to escalate its war in Ukraine.

“Russia would be making an incredibly serious mistake for it to use a tactical nuclear weapon,” Biden told reporters after getting his COVID-19 booster shot. “I’m not guaranteeing you that it’s a false flag operation yet. I don’t know. But it would be a serious, serious mistake.”

Russia over the weekend claimed Ukraine was preparing to use a “dirty bomb,” which typically refers to an explosive that would be used to spread radioactive material.

Ukraine has denied any attempt to use such a weapon, and officials from Ukraine, the U.S. and other allied countries have warned that Russia’s allegations could be laying the groundwork to take some type of escalatory action of its own.

“We’ve not seen any reason to adjust our own nuclear posture, nor do we have indications that Russia is preparing to use nuclear weapons,” Ned Price, a State Department spokesman, said Monday. “But we’ve heard these very concerning statements, and we wanted to send a very clear signal.”

Secretary of State Antony Blinken spoke Sunday with his Ukrainian counterpart. The two rejected Russia’s “transparently false allegations” and voiced confidence that “the world would see through any attempt by Russia to use this allegation as a pretext for escalation,” according to a readout of the call.

Biden has repeatedly warned Russia against the use of any type of nuclear weapon in its war against Ukraine, saying the consequences would be catastrophic without explicitly laying out how the United States would respond.

Russia invaded Ukraine without provocation in February, and Russian President Vladimir Putin has taken a number of steps in recent weeks that outside observers have seen as escalating the war, including the illegal annexation of Ukrainian territory, veiled references to nuclear weapons and the mobilization of hundreds of thousands of troops.