National Security Advisor of the United States, Jake Sullivan.

Washington says it will ‘respond decisively’ if Moscow uses nuclear weapons in Ukraine.

The United States has warned of “catastrophic consequences” if Moscow were to use nuclear weapons in Ukraine, after Russia’s foreign minister said regions holding widely-criticised referendums would get full protection if annexed by Moscow.

Votes in four eastern Ukrainian regions, aimed at annexing territory Russia has taken by force mostly since its invasion in February, were staged for a third day on Sunday. The Russian parliament could move to formalise the annexation within days.

By incorporating the four areas of Luhansk, Donetsk, Kherson and Zaporizhia into Russia, Moscow could portray attempts to retake them as attacks on Russia itself, a warning to Kyiv and its Western allies.

The Russian annexations raise the risk of a direct military confrontation between Russia and the NATO military alliance as Western arms are being used by Ukrainian troops.

US National Security Advisor Jake Sullivan said on Sunday that the US would respond decisively to any Russian use of nuclear weapons against Ukraine, and that it had spelled out to Moscow the “catastrophic consequences” it would face.

The latest US warning followed a thinly veiled nuclear threat made last Wednesday by Russian President Vladimir Putin, who said his country would use any weapons to defend its territory.

Foreign Minister Sergey Lavrov made the point more directly at a news conference on Saturday after addressing the United Nations General Assembly in New York City.

Asked if Russia would have grounds for using nuclear weapons to defend the annexed regions, he said Russian territory, including territory “further enshrined” in Russia’s constitution in the future, “is under the full protection of the state”.

In his speech, Lavrov sought to justify Russia’s seven-month war in Ukraine, repeating Moscow’s false claims that the elected government in Kyiv was illegitimately installed and filled with neo-Nazis.

He cast opposition to what Russia calls a “special military operation” as limited to the US and countries under its sway, although nearly three-quarters of states in the UN assembly voted to reprimand Russia and demand it withdraw.

Ukraine and its allies have dismissed the referendums as a sham designed to justify an escalation of the war and a mobilisation drive by Moscow after recent battlefield losses.

Russian news agencies quoted unidentified sources as saying the two houses of the Russian parliament could debate bills to incorporate the new territories as soon as Thursday. State-run RIA Novosti said Putin could address parliament in an extraordinary joint session Friday.