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.

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.

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.

Zelenskyy said Sunday that Russia is pushing ahead with referendums in parts of Ukraine it controls because President Vladimir Putin knows he is losing the war and must justify continuing to fight it.

Speaking on CBS's "Face the Nation," Zelenskyy said Putin plans on annexing the territories so he can justify continuing the invasion by claiming he is actually defending Russian territory. .

A monthlong Ukraine counteroffensive has driven Russia back from thousands of miles of Ukraine territory and compelled Putin to approve a "partial" military mobilization aimed at adding 300,000 soldiers to his battered military.

Lithuania, Latvia and Estonia, the three Baltic countries that were once a part of the Soviet Union and share a border with Russia, have voiced opposition to opening their doors to Russian refugees. 

A Finnish opposition leader, Petteri Orpo, said fleeing Russian military reservists were an “obvious” security risk and “we must put our national security first.”