In a presidential decree, the Russian president says he has recognised the independence of Zaporizhzhia and Kherson, with his country making a similar move in relation to Donetsk and Luhansk in February.
Russia is set to annex four occupied regions of Ukraine at a lavish Kremlin ceremony that follows a threat by President Vladimir Putin to use nuclear weapons in their defence.
Kremlin spokesperson Dmitry Peskov told reporters late on Thursday that the annexations would be formalised at the ceremony and Putin would deliver a "major" speech.
It comes after foreign leaders critical of Russia voiced their opposition to the plan, with US President Joe Biden saying the United States would "never, never, never" recognise Russian sovereignty over the territories — Kherson and Zaporizhzhia in Ukraine's south, and Donetsk and Luhansk in the east.
In a presidential decree issued on Thursday evening, Putin said he had recognised the independence of Zaporizhzhia and Kherson, paving the way for Moscow to claim the territories.
Russia recognised the independence of the two other regions it is preparing to annex — Donetsk and Luhansk — at the end of February.
The Kremlin's atomic threats have not deterred a sweeping Ukrainian counter-offensive, which has been pushing back Russian troops in the east.
Kiev's forces are on the doorstep of the Donetsk region town of Lyman, which Moscow's forces pummelled for weeks before capturing it this summer.
'No place in modern world'
Putin has blamed the conflict in Ukraine on the West and said simmering conflicts in the former Soviet Union were the result of its collapse.
The rhetoric built on his now famous phrase that the fall of the USSR was a tragedy, and he has recently suggested Moscow should again extend its influence over the former Soviet region.
The four regions' Kremlin-backed leaders who pleaded to Putin for annexation this week assembled in the Russian capital ahead of the ceremony.
Their almost simultaneous requests came after they claimed residents had unanimously backed the move in hastily organised referendums that were dismissed by Kiev and the West as illegal, fraudulent and void.
Ukraine said the only appropriate response from the West was to hit Russia with more sanctions and to supply Ukrainian forces with more weapons to keep reclaiming territory.
UN Secretary-General Antonio Guterres condemned the annexation plans as "a dangerous escalation" that "has no place in the modern world."
The UN Security Council will vote Friday on a resolution condemning the referendums, according to France, the council's current president, but it has no chance of passing due to Moscow's veto power.
Ukraine's President Volodymyr Zelenskyy called an "urgent" meeting of his national security council for Friday, his spokesperson said, after the Kremlin announced the timing of the annexation ceremony.
The four territories create a crucial land corridor between Russia and the Crimean peninsula, annexed by Moscow in 2014. Together, all five make up around 20 percent of Ukraine, whose forces in recent weeks have been clawing back ground.