Firmly Warns The Use Of Nuclear Weapons, US: Crosses This Limit, There Are Disastrous Consequences For Russia

US National Security Adviser Jake Sullivan. (Wikimedia Commons/The White House)

JAKARTA - The United States on Sunday warned of "catastrophic consequences" if Moscow used nuclear weapons in Ukraine, after Russia's foreign minister said the territory holding the referendum, which was widely criticized, would receive full protection if it was annexed by Moscow.

Voting in four regions of eastern Ukraine, aimed at annexing territory that had been forcibly taken by Russia, was held for a third day on Sunday. The Russian parliament could move to formalize the annexation within days.

By incorporating the Lugansk, Donetsk, Kherson and Zaporizhzhia regions into Russia, Moscow could portray attempts to retake them as an attack on Russia itself, a warning to Kyiv and its Western allies.

US National Security Adviser Jake Sullivan said on Sunday the United States would respond to any Russian use of nuclear weapons against Ukraine.

It said the "disastrous consequences" that Moscow would face from using this type of weapon had been explained.

"If Russia crosses this line, there will be disastrous consequences for Russia. The United States will respond decisively," Sullivan told NBC's "Meet the Press" television program.

The latest US warning follows a veiled nuclear threat made Wednesday by President Vladimir Putin, who said Russia would use any weapon to defend its territory.

Meanwhile, Foreign Minister Sergei Lavrov made a more direct point at a press conference on Saturday, following a speech at the UN General Assembly in New York, in which he repeated Moscow's false claims to justify the invasion that the elected government in Kyiv was illegitimately installed and filled with neo-Nazis.

Asked whether Russia would have any reason to use nuclear weapons to defend the annexed territories, Lavrov said Russian territory, including areas 'more enshrined' in Russia's future constitution, "is under the full protection of the state."