Russian President Vladimir Putin announced plans to annex four Ukrainian regions partially occupied by the Kremlin as part of its seven-month invasion of the neighboring country.

"This is the will of millions of people," Putin said during a speech in the St George's Hall of the Kremlin on Friday (September 30) via Reuters.

Russia will recognize the Ukrainian regions Luhansk, Donetsk, Kherson and Zaporizhzhia as Russian territories upon completion of the process, which will be the biggest annexation in Europe since World War II.

"People living in Luhansk, Donetsk, Kherson region and Zaporizhzhia region are becoming our compatriots forever," Putin said via Reuters.

Putin's announcement comes after several recent setbacks during Russia's invasion of Ukraine.

Pro-Russian officials confirmed that Kremlin troops were on the verge of encirclement in Lyman, while others believe that the battle may be an opening for Ukraine to take back the four territories recently declared part of Russia by Putin, Reuters reports.

Last week, Putin announced a partial military mobilization of 300,000 reservists and reiterated warnings to Western allies that he was ready to use nuclear weapons in Moscow's ongoing invasion of neighboring Ukraine, during a rare, prerecorded television announcement, Reuters reported.

Putin claimed that Ukraine -- a country he ordered troops to invade earlier this year -- "wants to destroy our country" during the announcement.

"If the territorial integrity of our country is threatened, we will use all available means to protect our people - this is not a bluff," Putin said via Reuters.

Russia's defense minister Sergei Shoigu confirmed that 300,000 reservists with prior military experience would be called up as part of the partial mobilization.

The decision marked Russia's first mobilization since World War II and the biggest escalation since invaded Ukraine in February.

Ukraine and its Western allies said Putin's call for more troops signified that his invasion of the neighboring country was failing.

"Absolutely predictable appeal, which looks more like an attempt to justify their own failure," Ukrainian Presidential Adviser Mykhailo Podolyak told Reuters. "The war is clearly not going according to Russia's scenario."

Allies vowed to continue their support of Ukraine amid Putin's announcement.

Putin announced his country would conduct military operations in Eastern Ukraine during an NBC News translation of a speech addressing the Russian population in Moscow on February 24.

The announcement appeared to serve as the final action ahead of an attack by Putin and the Russian military, which the U.S. and European allies to the neighboring Ukraine have attempted to prevent from taking place through diplomatic discussions.