Russia's unilateral declaration to annex four occupied Ukrainian regions has pushed Ukraine to formally apply to join the trans-Atlantic military alliance, as Moscow's seven-month-old war against Kyiv rages on, with fatalities reported Saturday in one of the newly annexed territories.

In the latest escalation of the conflict, Russian President Vladimir Putin on Friday declared the annexation of the self-proclaimed Donetsk People's Republic and the Luhansk People's Republic in Ukraine's east and the controlled parts of Zaporizhzhia and Kherson.

Ukrainian President Volodymyr Zelenskyy subsequently said on his official website that Ukraine is submitting a fast-track application to join the North Atlantic Treaty Organization.

"We are taking our decisive step by signing Ukraine's application for accelerated accession to NATO," said Zelenskyy, who has rebuffed Putin's call for cease-fire negotiations and signified his resolve to reclaim all the regions with the backing of the West.

Also Friday, Russian forces hit a convoy southeast of Zaporizhzhia, and local authorities reported that 25 civilians were killed, Britain's Defense Ministry tweeted Saturday.

"Russia is expending strategically valuable military assets in attempts to achieve tactical advantage and in the process is killing civilians it now claims are its own citizens," the ministry said.

Ukraine's bid is expected to be an uphill struggle, as all 30 NATO countries must ratify the applications to approve new members.

Ukraine follows in the steps of Finland and Sweden, which also submitted applications to join the NATO alliance earlier this year. The two countries were given a clear path to membership in June after Turkey withdrew its opposition.

The unilateral annexation followed recent referendums in the four regions where residents voted to join Russia, with the polls condemned by Ukraine and the West as illegitimate.

Russia's actions swiftly received an international backlash, with U.N. Secretary General Antonio Guterres saying on Twitter that the referendums "can't be called a genuine expression of the popular will," adding that any annexation of a state's territory by another state based on threat or use of force violates the U.N. Charter.

The U.S. administration of President Joe Biden also denounced Russia over its annexation and announced fresh sanctions in response to the violations.

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