The US and NATO on Tuesday condemned Kremlin-staged "referendums" in eastern Ukraine as voting concluded.
Pro-Moscow officials said all four occupied regions of Ukraine overwhelmingly voted to join Russia.
The votes, which Kyiv and its allies dismissed as a sham, were staged in Donetsk and Luhansk in the east and Kherson and Zaporizhzhia in the south.
99% of ballots cast in Donetsk and 98% in the Luhansk region voted in favour of joining the Russian Federation, according to Russian-backed election officials. In the Zaporizhzhia region 93% were in favour of annexation, with the Kherson region giving the lowest pro-annexation tally at 87%.
US Secretary of State Antony Blinken said the West would "never recognize" the pro-annexation votes, saying that the US "will impose additional swift and severe costs on Russia" for holding the "referendums."
NATO chief: 'These lands are Ukraine'
NATO chief Jens Stoltenberg said in a tweet that he spoke with Ukrainian President Volodymyr Zelenskyy and told him that the military alliance is "unwavering" in its support for Ukraine's sovereignty and right to self-defense.
"The sham referenda held by Russia have no legitimacy and are a blatant violation of international law. These lands are Ukraine," Stoltenberg said.
The outcome of the vote was never really in doubt, and the UK Defense Ministry said President Vladimir Putin planned to declare Russia’s annexation of four partially occupied regions shortly after the results were announced.
The polls were announced just days before voting began on Friday and were called amid an effective counteroffensive by Ukrainian forces, which has seen territory controlled by Russia shrink in recent weeks.
'Forced to vote while threatened by guns' Zelenskyy says
During his address to the UN Security Council, Ukrainian President Volodymyr Zelenskyy reiterated Kyiv's position that the votes on annexation were "sham" referendums.
"On the occupied territory of Ukraine people are forced to fill out some papers while being threatened by submachine guns," Ukraine's president said.
Zelenskyy claimed that the "alleged results of the sham referenda have been prepared well in advance." He called the annexation of Ukrainian territory a "brutal violation of the UN charter," adding that it was an "attempt to erase the norms of international law."
"Annexation is the kind of move that puts [Putin] alone against the whole of humanity," Zelenskyy declared, adding that talks with Moscow were not possible.
The Ukrainian head of state argued that Russian forces would conscript residents of annexed regions and "send them to fight against their own homeland."
Zelenskyy called for Russia to be either excluded or suspended from all international organizations, and demanded that Kyiv be given "clear and legally binding guarantees of collective security."
US to introduce resolution condemning 'referendums'
The United States said it would put forward a resolution at the UN Security Council condemning the "referendums."
The resolution will be introduced jointly with Albania.
"Russia’s sham referenda, if accepted, will open a Pandora's Box that we cannot close," US envoy Linda Thomas-Greenfield said.
"If Russia chooses to shield itself from accountability here in the Council, we will then look to the UN General Assembly to send an unmistakable message to Moscow," she added.
kb, sdi/wd, jsi (Reuters, AP, AFP, dpa)