President Joe Biden warned on Friday that Russian President Vladimir Putin would pay if he choses to use chemical weapons in the ongoing war in Ukraine.

“Russia would pay a severe price if they used chemical weapons,” he told reporters at the White House after a speech Friday.

The president did not detail his answer further, only telling reporters he would not speak in detail about U.S. intelligence warning that Putin would possibly use chemical weapons to trigger a “false-flag” attack in Ukraine.

“[W]e should all be on the lookout for Russia to possibly use chemical or biological weapons in Ukraine, or to create a false flag operation using them,” White House press secretary Jen Psaki wrote on social media Wednesday.

But the administration has been reluctant to say if the use of chemical weapons would be a “red line” requiring NATO military action.

The president said he does not want the United States fighting Russia in Ukraine, warning of the potential of another world war.

“Direct confrontation between NATO and Russia is World War III, something we must strive to prevent,” he said.

But Biden spoke aggressively against Putin, reminding him repeatedly that he “failed” by invading Ukraine.

“He hoped to dominate Ukraine without a fight. He failed. He hoped to fracture European resolve. He failed,” he said. “He hoped to weaken the transatlantic alliance. He failed. He hoped to split apart American democracies in terms of our positions. He failed.”