The United States Embassy in Moscow made an announcement telling all U.S. nationals to leave Russia "immediately."

"An increasing number of airlines are canceling flights into and out of Russia, and numerous countries have closed their airspace to Russian airlines," a U.S. Embassy & Consulates in Russia security alert read Sunday. "U.S. citizens should consider departing Russia immediately via commercial options still available."

The alert comes after Vladimir Putin put Russia's nuclear deterrence arsenal at the ready, pointing to the sanctions being put in place by the U.S. and other NATO countries.

"This particular decision is not in response to the situation in Ukraine, not even what is going on in Ukraine right now, but as a response to American readiness to apply to sanctions," Andrei Illarionov, senior fellow at the Center for Security Policy in Washington, D.C., told Sunday's "Wake Up America" on Newsmax. "The situation now is unbelievably grave."

The move is "incomparable" to any nuclear threat the world has faced to date, and the U.S. and NATO must respond in kind immediately, Illarionov told co-host Amanda Brilhante.

"If the United States will not respond with adequate reaction to this decision, it means not only Ukraine but the whole Europe and at some point the United States will be under nuclear threat, direct nuclear threat," Illarionov said in a dire warning.

"That is why United States and NATO countries need to respond immediately, right now, to this nuclear blackmail of Putin."

Illarionov made an urgent call for the Biden administration to activate the U.S. nuclear deterrence arsenal.

"That should be exactly the same," he said. "It should be putting all nuclear arsenal of the United States and NATO on the highest alert – at least equal response to what Putin has done."

Additionally, a security alert one week ago warned Americans to "carry proper identification, including a U.S. passport with a current Russian visa," and to be on alert "in locations frequented by tourists/Westerners."

The alert also tells Americans to have "evacuation plans that do not rely on U.S. government assistance."

"According to media sources, there have been threats of attacks against shopping centers, railway and metro stations, and other public gathering places in major urban areas, including Moscow and St. Petersburg as well as in areas of heightened tension along the Russian border with Ukraine," the alert read.

• Andrei Illarionov to Newsmax: Putin Threatening US With 'Nuclear Blackmail'
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