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The city of Aleppo, Syria, has largely fallen under government siege with humanitarian aid to thousands of people cut off and an estimated 150 civilians killed by Russian airstrikes, U.S. officials said Friday.

"In many respects, a siege of Aleppo has in fact happened," U.S. State Department spokesman John Kirby said Friday.

"We know that humanitarian aid routes have been cut off — certainly the most significant ones — and there continues to be bombardment from the air, obviously by Russian military aircraft," Kirby said. "And of course, the regime ground forces continue to squeeze the city."

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Aleppo, which was Syria's commercial hub before war broke out in 2011, is split between government and rebel control. Russia, which joined the conflict to aid Syrian President Bashar Assad in September, has in recent days intensified aerial attacks on the city.

Related: New Syria Crisis: Tens of Thousands Flee 'Siege of Aleppo'

Samantha Power, the U.S. ambassador to the United Nations, said Friday that at least 150 civilians, including three Syrian Red Crescent aid workers, have been killed in the recent Russian airstrikes.

At least 15,000 people have fled the city, according to the United Nations, and tens of thousands more are reportedly trying to reach the Turkish border.

The bombardment interrupted U.N.-led peace talks scheduled in Geneva this week.

Photos: Syrians Flee Aleppo as Government Takes City

"Instead of talking peace and practicing war, Russia must become part of the solution for Syria, a conflict that is causing catastrophic suffering and destabilizing the entire region," Power said.

"If it's just talk for the sake of talk and they are going to continue bombing, than no one is going to accept that. And we will know that in the course of the next days," U.S. Secretary of State John Kerry echoed.

Kerry said Russia and Assad were not in compliance with a U.N. Security Council resolution, which Russia voted for, that calls for access for humanitarian aid workers and an end to aerial bombardment of civilians.

"Russia is using free bombs, ... they are not precision bombs and there are civilians, including women and children being killed in large numbers as a consequence — hospitals have been hit, civilian quarters have been hit," Kerry said. "This has to stop."