Moments after Russia announced that it was annexing four regions in Ukraine that are under Kremlin control, Ukrainian President Volodymyr Zelenskyy said that his country was applying to join the North Atlantic Treaty Organization.

"We are taking our decisive step by signing Ukraine's application for accelerated accession to NATO," Zelenskyy said.

"Today, Ukraine is applying to make it de jure. Under a procedure consistent with our significance for the protection of our entire community, under an accelerated procedure," he added.

If NATO accepts Ukraine's request to join, member countries would be required to defend Ukraine in the ongoing war with Russia.

"It is here, in Ukraine, that the values of our Euro-Atlantic community have obtained real vital energy," Zelensky said. "The strength of the nation that fights for freedom, and the strength of the nations that help in this fight."

Leaders in Europe and the United States have denounced Russia's illegal decision to annex Donetsk, Luhansk, Kherson, and Zaporizhzhia following referendums over the weekend. They called the votes shams and vowed to put more sanctions in place to punish Russia.

"The United States unequivocally rejects Russia's fraudulent attempt to change Ukraine's internationally recognized borders," U.S. Secretary of State Antony Blinken said after announcing a host of sanctions on hundreds of Russians.

Blinken said that the sanctions are "a clear warning supported by G7 Leaders: We will hold to account any individual, entity, or country that provides political or economic support for Russia's illegal attempts to change the status of Ukrainian territory."