Speaking on CNN's "State of the Union" on Sunday, Thomas-Greenfield said the United States had not yet confirmed the allegations made on Saturday by the Mariupol city council via its Telegram channel.
"I've only heard it. I can't confirm it," she said. "But I can say it is disturbing. It is unconscionable for Russia to force Ukrainian citizens into Russia and put them in what will basically be concentration and prisoner camps."
Russia launched a large-scale invasion of Ukraine on February 24, igniting a conflict that has led to more than 900 civilian deaths and nearly 1500 injuries as of March 19, according to the UN human rights office.
Mariupol, a key connection to the Black Sea, has been a target since the start of the war, which Russian President Vladimir Putin calls a "special military operation" to demilitarise and "denazify" Ukraine.Â
Ukraine and the West say Putin launched an unprovoked war of aggression.
The Mariupol city council also said Russian forces bombed an art school on Saturday in which 400 residents had taken shelter, but the number of casualties was not yet known.
Reuters could not independently verify the claims by the city council. Russia denies targeting civilians.
The Russian Embassy in Washington did not immediately respond to a request for comment.
Ukrainian President Volodymyr Zelenskiy called on Saturday for comprehensive peace talks with Moscow.
The United States supports those attempts, Thomas-Greenfield said on Sunday, adding that the negotiations "seem to be one-sided," with little response from Russia.
Poland will formally submit a proposal for a peace-keeping mission in Ukraine at the next summit of the North Atlantic Treaty Organization, Polish Prime Minister Mateusz Morawiecki said on Friday.
When asked about the Polish proposal, Thomas-Greenfield reiterated President Joe Biden's commitment to refrain from sending US troops into Ukraine.
"Other NATO countries may decide that they want to put troops inside of Ukraine," she said. "That will be a decision that they have to make."
Turkey is also attempting to mediate a ceasefire in Ukraine, which NATO Secretary General Jens Stoltenberg praised during an interview on Sunday on NBC's "Meet the Press."
"Turkey is doing some real effort to try to facilitate, support talks between Russia and Ukraine," Stoltenberg said. "It's far too early to say whether these talks can lead to any concrete outcome."
When asked on Sunday whether a no-fly zone would be considered if Russia used chemical weapons in Ukraine, Stoltenberg reiterated concerns that such a step could escalate the conflict.
"Our allies support Ukraine," he said. "But at the same time it is extremely important we prevent this conflict from becoming a full-fledged war between NATO and Russia that will cause much more damage, much more death, destruction than what we are seeing now in Ukraine."
