A U.S. Department of Defense official said Western intelligence believes Russia is trying to decapitate the government of Ukraine and seek to install their own officials to take charge of the country.
Officials said that there is fighting within 20 miles of the center of Kyiv, the Ukrainian capital, as well as Kharkiv, one of the largest cities in Ukraine.
"These three axes are … clearly designed to take key population centers," the official said.
On early Thursday, Russia carried out airstrikes and launched missiles at Ukrainian military positions, according to officials. After those attacks, Russian troops, tanks, and artillery started moving into Ukraine, although the Ukrainian armed forces have been able to repel some of Moscow's assaults.
They used about 75 fixed-wing heavy and medium bombers to carry out their assault, attacking air defense positions and military bases, U.S. officials said. It's not clear how many died.
U.S. Defense officials asserted that Russian President Vladimir Putin made a decision to invade Ukraine even before diplomatic talks were held. Western officials for weeks have warned that Russia would launch an attack on its neighbor, including the capital Kyiv, although Ukraine's leadership asserted that they had believed Putin wouldn't attack and largely dismissed Western intelligence reports.
An adviser to the Ukrainian presidential office said Russian forces had captured the Chernobyl former nuclear power plant, just 90 km (60 miles) north of the capital, and Hostomel airport in the Kyiv region, where paratroopers had earlier been landed.
Heavy exchanges of fire were also taking place in the regions of Sumy and Kharkiv in the northeast and Kherson and Odessa, home to a populous city and Ukraine's most important seaport, in the south.
President Joe Biden on Thursday announced more sanctions against Russian assets and threatened more, but he stressed that no U.S. troops would be deployed in Ukraine.
"This is a premeditated attack," he told reporters at the White House. "Putin is the aggressor. Putin chose this war. And now he and his country will bear the consequences."
According to the White House, the measures restrict Sherbank from transactions made in American dollars while limiting Moscow's capabilities to import goods.
And Ukrainian President Volodymyr Zelensky urged Ukrainians to defend their country and promised that weapons would be given to anyone prepared to fight.
"What we have heard today are not just missile blasts, fighting, and the rumble of aircraft. This is the sound of a new Iron Curtain, which has come down and is closing Russia off from the civilized world," Zelensky said.