The Russian parliament's upper chamber has ratified agreements on the admission of the Luhansk, Donetsk, Kherson and Zaporizhzhia regions into the Russian Federation.

The Federation Council on Tuesday voted unanimously to incorporate the four regions of Ukraine, following ratification by the State Duma, Russia's lower house, the previous day.

President Vladimir Putin's final signature will be needed to formalize the incorporation which Moscow declared after holding what it called referendums in the areas. 

Putin is "likely" to sign the documents during the course of the day, Kremlin spokesman Dmitry Peskov said on Tuesday.

The referendum results released last week showed over 90 percent of voters in each region voting in favor of joining the Russian Federation.

The results were however rejected by Kyiv and Western governments who said the votes breached international law.

On Tuesday, Ukrainian President Volodymyr Zelenskyy signed a decree formally declaring the prospect of any talks with Putin "impossible," though he left the door open to talks with Russia.

In response, the Kremlin said talks need to be joined by both sides and that Russia was committed to achieving its aims through peaceful means before it launched its "special military operation."

For now, Russia can only wait for the current or the future Ukrainian president to change their position, it added.

Moscow is yet to formally designate the borders of the new regions. Peskov said on Monday consultations were ongoing regarding the borders of the Zaporizhzhia and Kherson regions.

Fighting continued in the regions Russia is expected to formally absorb, with Ukraine on Monday reportedly taking territory tens of kilometers behind the previous frontlines in the southern Kherson region.

Meanwhile Russian forces control around 60 percent of the Donetsk region and 70 percent of Zaporizhzhia, while recent Ukrainian advances have also pushed the frontlines back into Luhansk, a region over which Russian forces claimed full control in July.

(With input from Reuters)