UNITED NATIONS, Sept 25 (Reuters) - Russia's top diplomat on Saturday (Sep 24) said regions of Ukraine where widely-derided referendums are being held would be under Russia's "full protection" if they are annexed by Moscow, amid fears Russia could further escalate the conflict and even use nuclear weapons.

Foreign Minister Sergei Lavrov, addressing the UN General Assembly and the world's media in New York, attempted to justify Russia's February invasion of its neighbor, repeating Moscow's false claims that the elected government in Kyiv was illegitimately installed, filled with neo-Nazis and oppressed Russian speakers in the country's east.

Russia on Friday launched referendums in four eastern Ukrainian regions aimed at annexing territory it has taken by force. Kyiv said residents were being coerced into voting and were not allowed to leave the regions during the four-day vote, which Western nations dismissed as a sham designed to justify an escalation of the seven-month old war.

"Following those referendums, Russia of course will respect the expression of the will of those people who for many long years have been suffering from the abuses of the neo-Nazi regime," Lavrov said at a news conference after he addressed the assembly.

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

"All of the laws, doctrines, concepts and strategies of the Russian Federation apply to all of its territory," he said, also referring specifically to Russia's doctrine on the use of nuclear weapons.

The comments came after an explicit warning on Thursday by former President Dmitry Medvedev, an ally of Russian President Vladimir Putin, that any weapons in Moscow's arsenal, including strategic nuclear weapons, could be used to defend territories incorporated into Russia.

Ukraine's Foreign Minister Dmytro Kuleba said Lavrov's comments, and Putin's earlier statement when he said he was not bluffing about using nuclear weapons, were "irresponsible" and "absolutely unacceptable".

"Ukraine won’t give in. We call on all nuclear powers to speak out now and make it clear to Russia that such rhetorics put the world at risk and will not be tolerated," Kuleba wrote on Twitter.

Russia accuses the United States and others of being parties to the conflict because they are sending weapons to help Ukraine defend itself. The likely annexation of Ukrainian territory raises the question of how Russia might respond to the use of Western weapons in those regions.

Ukraine also requested an urgent UN Security Council meeting over the referendums, calling for Russia to be "held accountable for its further attempts to change Ukraine’s internationally recognised borders in a violation of the UN Charter," foreign affairs ministry spokesperson Oleg Nikolenko said on Twitter.