The U.S. has warned the Kremlin of the devastating consequences Russia would face should nuclear weapons be used in Ukraine, national security adviser Jake Sullivan said Sunday.

"If Russia crosses this line, there will be catastrophic consequences for Russia," Sullivan said on NBC's "Meet the Press." "The United States will respond decisively. In private channels, we have spelled out in greater detail exactly what that would mean."

Ukraine President Volodymyr Zelenskyy said he does not believe Putin was bluffing when he said last week that he would be justified in using any force necessary — implying tactical nuclear weapons are not off the table.

"He wants to scare the whole world," Zelenskyy said. "I think the world is deterring it and containing this threat. We need to keep putting pressure on him."

Sullivan also said Putin must be held accountable for the war crimes and atrocities Russian troops have committed in Ukraine, but acknowledged diplomacy will ultimately be required to end the conflict. That almost certainly won't happen until Putin's ready to negotiate.