The United States will respond decisively if Russia uses nuclear weapons against Ukraine, U.S. National Security Adviser Jake Sullivan said today, and that Washington has explained to Moscow authorities the "catastrophic consequences" they will face.

In response to the Russian-Ukrainian war, Russian President Vladimir Putin ordered Russia's first mobilization since World War II on September 21 and warned Western countries that he was not bluffing when he said he was ready to use nuclear weapons to defend Russia.

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"If Russia crosses this line, it will be disastrous for Russia. The United States will respond decisively," Sullivan told NBC's "Meet the Press."

In his talk today, Sullivan did not describe the type of response the U.S. plans to respond to, but said the U.S. had "explained in more detail what that means" to Moscow authorities under the table.

He also said that the United States has been in frequent and direct contact with Russia, including over the past few days to discuss the situation in Ukraine and Putin's actions and threats.

U.S. President Joe Biden also delivered a speech at the United Nations General Assembly in New York on the 21st, accusing Putin of recklessly disregarding his responsibility to prohibit the proliferation of nuclear weapons and "blatantly making nuclear threats against Europe."