US President Joe Biden has warned Russia against using nuclear weapons in Ukraine, saying it would be an “incredibly serious mistake”.

The Biden administration had earlier said Russia had given notice that it intended to stage routine drills of its nuclear capabilities while Ukraine's nuclear energy operator claimed its neighbor was performing some secret work at Europe's largest nuclear power plant.

“Let me just say: Russia would be making an incredibly serious mistake if it uses a tactical nuclear weapon,” Biden told reporters at the White House on Tuesday.

He was responding to a question on whether Russia was preparing to deploy a “dirty bomb” or a nuclear weapon.

“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,” Biden stressed.

White House Press Secretary Karine Jean-Pierre said the President was clear about what he said. “He said it again today. It would be a major mistake for Russia to use nuclear weapons in Ukraine, leading to severe consequences. As for the potential use of a dirty bomb, look, Russia is pushing transparently false allegations,” she claimed.

“You heard me say that. You heard my colleague over at the State Department say yesterday that Ukraine is preparing to use a dirty bomb on its own territory. We are calling that false, false allegations by Russia.

“So, we must take this seriously… in the past, we have seen Russia use allegations as a pretext to escalate,” she said.

Jean-Pierre, however, said the US had not seen Russia make any preparations on those lines but stressed that it will continue to monitor the situation “very, very closely”.

Warning of consequences for Russia, she said, “I'm certainly not going to lay them out here. And, look, you know, we have communicated directly with them. I'm just not going to get ahead of any conversations on what we've laid out for the Russians.”

Responding to a question on peace negotiations between Russia and Ukraine, Jean-Pierre said it was up to Ukrainian President Volodymyr Zelenskyy to negotiate.

“…He (Zelenskyy) will make that decision for his country,” Jean-Pierre said.

(Inputs from PTI)?