Polls described as "simulacra" by kyiv and the West.
Russian annexation "referendums" began on Friday in areas of Ukraine wholly or partly controlled by Moscow, Russian news agencies reported.
Read alsoWar in Ukraine: annexation referendums, Putin's new weapon
These votes, which began at 5 a.m. GMT, are to be held until September 27 in the pro-Russian separatist regions of Donetsk and Lugansk (east), and in areas under Russian occupation in the regions of Kherson and Zaporizhia (south), in the midst of Moscow's military offensive against Ukraine.
The integration of these regions into Russia, following referendums whose outcome is not in doubt, would represent a major escalation in the conflict.
The territories of Donetsk and Lugansk, which together form the Donbass region, are partly controlled by pro-Russian separatists who rose up in 2014 against kyiv with the support of Moscow.
Moscow has already annexed the Ukrainian peninsula of Crimea (south) in 2014, an attachment ratified by a referendum which had been preceded by a military intervention and which is denounced as illegal by kyiv and the West.
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