Russia currently occupies two regions in eastern Ukraine. Local authorities expressed over the past weekend plans to hold referendums on formal membership in Russia. An ally of President Vladimir Putin said the votes would forever alter the geopolitical landscape in Moscow's favor.

The self-proclaimed Russian-backed Luhansk People's Republic (LPR) and neighboring Donetsk People's Republic (DPR) said referendums on the table would be held between September 23 and 27, according to Reuters.

"I ask you as soon as possible, in case of a positive decision in the referendum - about which we have no doubts - to consider the DPR as part of Russia," the head of state of the Donetsk People's Republic, Denis Pushilin, wrote on a social network.

On Tuesday morning, Russian officials deployed in the southern Kherson region, where Moscow forces control some 95% of the territory, said they had also decided to hold a referendum. Pro-Russian authorities in parts of Ukraine's Zaporizhia region were expected to follow suit.

The latest reports indicate a strong counter-offensive by the Ukrainians against the Russians, and formalization of membership in Russia could aggravate the situation with the West.

Ukraine and the United States said that such referenda would be an illegal sham and made it clear that they and many other countries would not recognize the results.

Dmitry Medvedev, a former president who is currently vice president of the Security Council, suggested before the announcements that the outcome of such votes would be irreversible and would give carte blanche to Moscow - which has the world's largest stockpile of nuclear weapons - to defend what it would legally consider its own territory.

"The invasion of Russian territory is a crime that allows you to use all self-defense forces," Medvedev said in a Telegram post. "That is why these referendums are so feared in Kyiv and in the West."

No future Russian leader would be able to constitutionally reverse its outcome, he added.