The United States 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.

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.

“If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively,” Sullivan told NBC’s Meet the Press news programme.

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”. (Al Jazeera)