Officials initially resisted such a move, citing escalation concerns and uncertainty over how Putin would respond. Some of those officials, including in the White House, the Pentagon and the State Department, feared lethal retaliation on U.S. military and diplomatic personnel and attacks on NATO allies.

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World leaders and analysts expressed doubts that the change to Russia's nuclear doctrine amounted to much more than a new and more intense effort to deter the West.

A decree signed by the Russian leader, though long planned, came days after President Biden authorized the use of U.S.-supplied missiles by Ukraine for strikes inside Russia.

Russia's Dmitry Medvedev warned of nuclear war if the U.S transfers nuclear weapons to Ukraine: "The consequences are obvious."