The Russian-backed rebel region of Luhansk in eastern Ukraine said on Sunday that it may hold a referendum on joining Russia, prompting Kyiv to warn that such a vote would have no legal basis and would result in a stronger international response.

President Vladimir Putin recognized the Ukrainian rebel regions of Luhansk and Donetsk as independent states three days before ordering the February 24 invasion of Ukraine, despite the fact that the rest of the world considers them to be part of Ukraine.

Ukraine, which claims to be fighting for its survival against what it sees as an imperial-style land grab by Russia, has repeatedly stated that it will never agree to Russia’s annexation of its territory – the most difficult part of peace talks with Moscow.

“I believe that a referendum will be held on the republic’s territory in the near future,” Leonid Pasechnik, the leader of the self-proclaimed Luhansk People’s Republic, was quoted as saying by the region’s news outlet.

“The people will exercise their ultimate constitutional right by voting on whether or not to join the Russian Federation.”

Ukraine said such a referendum in occupied Ukrainian territory would have no legal basis and would face a strong response from the international community, deepening Russia’s global isolation.

“All fake referendums in the temporarily occupied territories are null and void and will have no legal validity,” Ukraine’s foreign ministry spokesperson Oleg Nikolenko said in a statement to Reuters.

“Instead, Russia will facе an even stronger response from the international community, further deepening its global isolation.”

Putin says what he casts as a “special military operation” was needed to defend Russia against the United States which he said was arming Ukraine and developing it into an “anti-Russia” with the aim of admitting it into the NATO military alliance.

Russia also says the operation was needed to defend Russian speaking populations against persecution from Ukraine’s government which it says is a puppet controlled by Washington.

Ukraine says such claims are simply a pretext for an occupation and that Moscow does not understand that Ukraine is a sovereign country.

After Russian forces in 2014 took control of Crimea – first annexed by Russia in 1783 – a referendum on joining Russia was held. Voters chose overwhelmingly to join Russia. Ukraine said the referendum was illegal and that Crimea is part of Ukraine.

The West imposed sanctions on Russia over the 2014 annexation of Crimea. Moscow says the majority of the people of Crimea wanted to join Russia.