Separatist leaders in four Russian-controlled regions of Ukraine have announced plans to begin voting to become integral parts of Russia.
The referendums in the Donetsk, Luhansk, Kherson and partly Russian-controlled Zaporizhzhia regions will start on Friday and last until Tuesday.
It comes after a close ally of Russian President Vladimir Putin said the votes were needed as Moscow loses ground in the war.
Former president Dmitry Medvedev said folding regions into Russia itself would make redrawn frontiers "irreversible" and allow Moscow to use "any means" to defend them.
The votes in territory Russia already controls are expected to go Moscow's way but are unlikely to be recognised by western governments.
Luhansk and Donetsk form much of the Donbas region, which has seen separatist fighting since 2014 and is a key objective for Mr Putin's invasion.
In Donetsk, separatist leader Denis Pushilin said the "long-suffering people of the Donbas have earned the right to be part of the great country that they always considered their motherland".
He said the vote will help "restore historic justice that millions of the Russian people were waiting for".
Kremlin spokesman Dmitry Peskov has said there are no prospects for a diplomatic settlement amid Ukraine's counter-offensive.
Mr Medvedev, the deputy head of Russia's security council, which is chaired by Mr Putin, said votes in separatist regions are important to protect their residents and "restore historic justice" and would "completely change" Russia's future trajectory.
"After they are held and the new territories are taken into Russia's fold, a geopolitical transformation of the world will become irreversible," said Mr Medvedev, who served as Russia's president from 2008 to 2012.
"An encroachment on the territory of Russia is a crime that would warrant any means of self-defence," he said, adding Russia would enshrine the new territories in its constitution so no future leader could hand them back.
"That is why they fear those referendums so much in Kyiv and in the West," Mr Medvedev said. "That is why they must be held."
It comes as Russian stocks plunged to their lowest in a month as Moscow reignited fears it would implement martial law with new legislation and its plans to hold referendums in Russian-controlled regions of Ukraine.
Russia's parliament approved a bill to toughen punishments for several crimes such as desertion, damage to military property and insubordination if they are committed during military mobilisation or combat situations.