The US said it will have to continue to stem Putin's actions in the strongest possible way. (File)
President Vladimir Putin's order to put Russia's deterrence forces, which include nuclear arms - on high alert is unacceptable escalation, US Ambassador to the United Nations Linda Thomas-Greenfield said on Sunday.
"It means that President Putin is continuing to escalate this war in a manner that is totally unacceptable and we have to continue to stem his actions in the strongest possible way," Thomas-Greenfield said in interview with CBS "Face the Nation."
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