Putin escalating in unacceptable manner with nuclear high alert - U.S. ambassador to U.N.

President Vladimir Putin's order to put Russia's deterrence forces, which include nuclear arms - on high alert is unacceptable escalation, U.S. 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."