Russia-Ukraine updates: US to ban Russian carriers from its airspace

As reports of explosions came in from multiple Ukrainian cities, NATO Secretary General Jens Stoltenberg issued a statement condemning “Russia’s reckless and unprovoked attack.”

Booms and explosions have been heard in Kyiv -- Ukraine’s capital, which is in the north-central part of the country -- as well as Dnipro in central Ukraine and Odessa in the southwest.

“I strongly condemn Russia’s reckless and unprovoked attack on Ukraine, which puts at risk countless civilian lives,” Stoltenberg said. “Once again, despite our repeated warnings and tireless efforts to engage in diplomacy, Russia has chosen the path of aggression against a sovereign and independent country.”

Stoltenberg said NATO allies would meet to talk about consequences of Russia’s “aggressive” actions.

“This is a grave breach of international law and a serious threat to Euro-Atlantic security,” he said. “I call on Russia to cease its military action immediately and respect Ukraine’s sovereignty and territorial integrity.”

President Joe Biden released a statement calling Russian President Vladimir Putin’s military operation in Ukraine an "unprovoked and unjustified attack by Russian military forces."

"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," Biden said. "Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way. The world will hold Russia accountable."

Biden said he would be monitoring the situation from the White House.

On Thursday, he will meet with his "G7 counterparts" -- leaders from Canada, France, Germany, Italy, Japan and the United Kingdom -- and then will give an address to announce "further consequences" on Russia "for this needless act of aggression against Ukraine and global peace and security," he said.

Additionally, Biden said, the U.S. will coordinate with NATO allies "to ensure a strong, united response that deters any aggression against the Alliance."

As President Vladimir Putin announced his invasion of Ukraine on Russian state TV, President Joe Biden's envoy at the U.N. was making a plea to Moscow to halt any action.

"This is a perilous moment, and we are here for one reason and one reason only: to ask Russia to stop," Ambassador Linda Thomas-Greenfield said at the special session. "Return to your borders. Send your troops, and your tanks, and your planes back to their barracks and hangars. And send your diplomats to the negotiating table."

"Back away from the brink, before it is too late," she added, though it already appears to be too late.

Thomas-Greenfield said she spoke with Biden before Wednesday night's meeting, saying he wanted to make clear the U.S. and its allies "will continue to respond to Russia's actions with unity, clarity and conviction."

Thomas-Greenfield said "all parties are not culpable here. There's no middle ground. ... Russia is the aggressor here."

Like other U.S. officials, and Ukrainian President Volodymyr Zelenskyy, she tried to appeal to the Russian people: "Everyday Russians should be asking themselves, right now, how many Russian lives Putin will sacrifice for his cynical ambitions?"

She ended by quoting the Ukrainian foreign minister, saying, "The people of Ukraine are counting on us. Let’s not let them down."

-ABC News' Conor Finnegan

Explosions have been heard in Kyiv, the capital of Ukraine.

-ABC News’ Kirit Radia