Votes will take place in the self-proclaimed Donetsk and Lugansk republics that Russian President Vladimir Putin recognized as independent shortly before sending troops to Ukraine in February, according to officials and news agencies.
A vote will also be held in the southern Kherson region that Moscow's troops captured in the early days of Russia's offensive in Ukraine.
The Russian-held Ukrainian region of Zaporizhzhia, home to Europe's largest nuclear plant, will join the other occupied territories in holding votes on becoming part of Russia, a pro-Moscow official said Tuesday.
"Today I signed an order on the holding of a referendum on the territorial allegiance of the region" from September 23 to 27, the head of the Russian-installed administration in Zaporizhzhia, Yevgeny Balitsky, said on Telegram, after similar votes were announced in the eastern regions of Donetsk and Lugansk and in the southern region of Kherson.
Their integration into Russia would represent a major escalation of the conflict in Ukraine as Moscow would be able to say it is defending its own territory from Ukrainian forces.
"The People's Council ruled... to set the days of the referendum for September 23 to September 27," separatist official Denis Miroshnichenko said as quoted by the Lugansk news portal.
Shortly after, the official Donetsk news agency announced that a referendum would be held in its territory on the same dates.
In a separate statement, the leader of the Donetsk rebel group Denis Pushilin asked Putin to consider the region becoming part of Russia "as soon as possible".
"The long-suffering people of Donbas deserve to be part of the Great Country, which they always considered their Motherland," Pushilin said on social media.
"I am sure that the entry of the Kherson region into the Russian Federation will secure our territory and restore historical justice," the Moscow-installed head of that region, Vladimir Saldo, said in a statement announcing the September vote.
He added that it was a "necessary decision in the face of constant acts of terror by the armed forces of Ukraine and NATO member countries that supply weapons to kill civilians on our soil".
Large parts of the industrial Donbas area have been controlled by Moscow-backed separatists since 2014, after nationwide demonstrations ousted Ukraine's Kremlin-friendly president.
Russia at the time annexed the Crimean peninsula from Ukraine with a vote that was criticized by Kyiv and the West, which imposed sanctions in response.
Kyiv countered the "annexation vote" announcements Tuesday, vowing to "eliminate" Russian threats against the war-torn country, after pro-Moscow separatists said they would hold referenda to join Russia later this week.
"Ukraine will solve the Russian issue. The threat can be eliminated only by force," said the Ukraine presidency's chief of staff, Andriy Yermak.
