Former President George W. Bush said Thursday that the Russian war on Ukraine is the most severe threat to European security since World War II as Russian President Vladimir Putin lurches Europe, and the world, toward another Cold War. 

"Russia’s attack on Ukraine constitutes the gravest security crisis on the European continent since World War II. I join the international community in condemning Vladimir Putin’s unprovoked and unjustified invasion of Ukraine," Bush said. "The American government and people must stand in solidarity with Ukraine and the Ukrainian people as they seek freedom and the right to choose their own future."

George W. Bush

In this Oct. 19, 2017, file photo, former U.S. President George W. Bush speaks at a forum sponsored by the George W. Bush Institute in New York. (AP)

RUSSIA INVADES UKRAINE: LIVE UPDATES

Russia began a full-scale invasion of Ukraine from multiple fronts Wednesday evening U.S. time. Putin declared a "special military operation" in the country in a pre-recorded video from earlier this week that was broadcast early Thursday morning Russian time. 

Air raid sirens and explosions could be heard in Kyiv, Ukraine's capital, throughout Thursday. Massive plumes of smoke were also visible in the area. 

Vladimir Putin

In this image made from video released by the Russian Presidential Press Service, Russian President Vladimir Putin addressees the nation in Moscow, Russia, Thursday, Feb. 24, 2022.  (Russian Presidential Press Service via AP)

Ukraine's forces are massively outgunned, and the country is expected to fall in the coming days. But a punishing insurgency against Russian occupation is also expected. Ukraine's foreign ministry has already shared photos of Russian soldiers it says it took prisoner. 

RUSSIA INVADES UKRAINE IN LARGEST EUROPEAN ATTACK SINCE WWII

Bush's comments came shortly after United Kingdom Prime Minister Boris Johnson issued similar comments saying his country and its allies stand with Ukraine. 

President Biden

President Biden speaks in the Eisenhower Executive Office Building's South Court Auditorium at the White House in Washington, Nov. 23, 2021. (REUTERS/Evelyn Hockstein)

"Our mission is clear," Johnson said. "Diplomatically, politically, economically and eventually militarily, this hideous and barbaric venture of Vladimir Putin must end in failure."

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"We will work with them for however long it takes to ensure that the sovereignty and independence of Ukraine is restored," Johnson added. 

Volodymyr Zelenskyy, Ukraine President

Ukrainian President Volodymyr Zelenskyy gestures as he addresses lawmakers in Kyiv, Ukraine, Wednesday, Dec.1, 2021.  (AP Photo/Efrem Lukatsky)

President Biden Wednesday condemned Russia's war as "an unprovoked and unjustified attack" and promised to levy tough sanctions against Russia. Biden also said he spoke with Ukrainian President Volodymyr Zelenskyy early Thursday and said, "We will continue to provide support and assistance to Ukraine and the Ukrainian people."

Biden is expected to deliver further remarks at noon.

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"We cannot tolerate the authoritarian bullying and danger that Putin poses," Bush added. "Ukraine is our friend and democratic ally and deserves our full support during this most difficult time."

Fox News' Trey Yingst and Peter Doocy contributed to this report.