The United States has warned of “catastrophic consequences” if Moscow were to deploy strategic nuclear weapons in Ukraine.

US National Security Adviser Jake Sullivan said the United States would respond decisively to any Russian use of nuclear weapons against Ukraine and that it had spelled out to Moscow what would happen.

“If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively,” Sullivan told NBC.

The warning follows an announcement by Russia’s Foreign Minister that regions of Ukraine holding widely-criticised referendums will receive 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.

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.

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

Foreign Minister Sergei Lavrov made the point more directly at a news conference on Saturday after addressing the UN General Assembly in New York.

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”.

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.

British Prime Minister Liz Truss said the UK and its allies should not heed threats from Putin, who had made what she called a strategic mistake as he had not anticipated the strength of reaction from the West.