"This is a pattern that we've seen from President Putin through the course of this conflict, which is manufacturing threats that don't exist in order to justify further aggression," White House press secretary Jen Psaki said on ABC when asked about the announcement from Moscow.

The American ambassador to the United Nations, Linda Thomas-Greenfield, said she condemned Putin's step in the strongest terms.

"It means that President Putin is continuing to escalate this war in a manner that is totally unacceptable," Thomas-Greenfield said in an interview on CBS.

Moscow has the world's second-largest arsenal of nuclear weapons and a huge cache of ballistic missiles which form the backbone of the country's deterrence forces.

Putin said he was putting Russia's nuclear forces on high alert because, he said, Western countries are taking "unfriendly" steps against his country.

The US and its allies have unleashed sweeping and punishing economic sanctions against Russia because of its invasion of Ukraine.

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