US National Security Adviser Jake Sullivan warned on Sunday that the United States would respond decisively to any Russian use of nuclear weapons against Ukraine and has spelled out to Moscow the "catastrophic consequences" it would face.

"If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively," Sullivan told NBC's "Meet the Press" program, as quoted by Reuters.

Sullivan did not describe the nature of the planned US response in his comments on Sunday but said the United States has privately to Moscow "spelled out in greater detail exactly what that would mean."

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

The comments come after Russian President Vladimir Putin issued a thinly veiled nuclear threat in a speech on Wednesday in which Putin also announced his country's first wartime military mobilization since World War Two.

US President Joe Biden, in his speech at the United Nations General Assembly, accused Putin of making "overt nuclear threats against Europe" in reckless disregard for nuclear nonproliferation responsibilities.

Earlier, in an interview with CBS’ “60 Minutes”, Biden urged Putin to not use tactical nuclear or chemical weapons in the wake of setbacks in Ukraine.

Asked what he would say to Putin if he was considering using such weapons, Biden replied, “Don’t. Don’t. Don’t. It would change the face of war unlike anything since World War Two.”

Biden also said the US response would be “consequential,” but declined to give detail.

Sullivan said on Sunday, "Putin remains intent ... on wiping out the Ukraine people that he does not believe have a right to exist. So he's going to keep coming and we have to keep coming with weapons, ammunition, intelligence and all the support we can provide."

(Israel National News' North American desk is keeping you updated until the start of Rosh Hashanah in New York. The time posted automatically on all Israel National News articles, however, is Israeli time.)