Russia's Foreign Minister said regions holding widely-criticised referendums would get full protection if annexed by Moscow, and the United States warned of the consequences if Moscow were to use nuclear weapons in Ukraine.
Votes in four eastern Ukrainian regions aimed at annexing territory Russia has taken by force most recently since its invasion in February, were held on Sunday for a third day. The Russian parliament could formalise the annexation within a few days.
By incorporating the four areas of Luhansk, Donetsk, Kherson and Zaporizhzhia into Russia, Moscow could portray attacks as an attack on Russia itself, a warning to Kyiv and its Western allies.
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 US National Security Adviser Jake Sullivan said on Sunday that the United States 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. Sullivan told NBC's Meet the Press that the United States will respond decisively.
The latest US warning came after President Vladimir Putin made a thinly veiled nuclear threat last Wednesday, which Putin said Russia would use any weapons to defend its territory.
Foreign Minister Sergei Lavrov spoke to the UN General Assembly in New York on Saturday.
Asked whether Russia would have grounds for using nuclear weapons to defend the annexed regions, Lavrov said that Russian territory, including territory that is now enshrined in Russia's constitution in the future, is under the full protection of the state.
He sounded against Russia calling it a special operation limited to the United States and countries under its sway, although nearly three-quarters of states in the assembly voted to reprimand Russia and demand it withdraw.
Ukraine and its allies have dismissed the referendums as a sham intended to justify an escalation of the war and mobilization drive by Moscow after recent battlefield losses.
British Prime Minister Liz Truss said Britain and its allies should not heed threats from Putin, who had made a strategic mistake as he had not anticipated the strength of reaction from the West.
We should not be listening to his sabre-rattling and his bogus threats. Truss told CNN that "what we need to do is to continue to put sanctions on Russia and continue to support the Ukrainians."