Russian President Vladimir Putin announced early Thursday local time that military operations had begun in Ukraine, kicking off a long-feared attack on their European neighbor.

Earlier Wednesday, Ukraine had taken steps to brace for a possible Russian invasion, declaring a nationwide state of emergency and calling up 36,000 military reservists.

U.S. President Joe Biden said a day earlier that the world is witnessing "the beginning of a Russian invasion of Ukraine" as he announced new economic sanctions on Russia, after weeks of escalating tensions in the region.

Biden's remarks followed a fiery address from Putin to the Russian public on Monday evening, when the leader announced he was recognizing the independence of two Russia-backed separatist areas in eastern Ukraine's Donbas region -- the self-proclaimed People’s Republics of Donetsk and Luhansk -- which prompted a set of sanctions from Western countries, including Germany halting approval of a major gas pipeline from Russia.

While the United States says some 190,000 Russian troops and pro-Russian separatist forces are estimated to be massed near Ukraine's borders, Russia has blamed Ukraine for stoking the crisis and reiterated its demands that Ukraine pledges to never join NATO.