US: "catastrophic consequences" if Russia uses nuclear weapons

"The United States will respond decisively. Now in private channels, we have spelled out in greater detail exactly what that would mean," he stressed.

The US is planning "for every contingency. And we will do what is necessary to deter Russia from taking this step. And if they do, we will respond decisively," Sullivan affirmed in statements to ABC network.

"We want to lay down the principle that there will be catastrophic consequences, but not engage in a game of rhetorical tit for tat," he made clear.

Meanwhile, British Prime Minister Liz Truss told CNN that Russian President Vladimir Putin "made a strategic mistake invading Ukraine." "I think he has been outsmarted by the Ukrainians.

We've seen the Ukrainians continue to push back against the Russian offensive. And I think he didn't anticipate the strength of reaction from the free world.

"We should not be listening to his saber-rattling and his bogus threats. Instead, what we need to do is continue to put sanctions on Russia and continue to support the Ukrainians," Truss noted.

She stressed that "if Putin is allowed to succeed, this wouldn't just send a terrible message in Europe and of course, huge threats to the Ukrainian population themselves, but it also would send a message to other authoritarian regimes around the world that it's somehow acceptable" to invade a sovereign nation." 

For his part, Ukrainian President Volodymyr Zelensky said that Putin's threat of nuclear weapons use "could be a reality." "Maybe yesterday it was bluff. Now, it could be a reality," he said in an interview on CBS network. "I don't think he's bluffing. I think the world is deterring it and containing this threat. We need to keep putting pressure on him and not allow him to continue." He added that the referenda that are happening in Eastern Ukraine "can lead to very tragic moments." "I see other threats when they complete, if they succeed with these referenda. The ballots have been- had been already prepared," he stressed.

"The Russian government can officially announce that the referendum had been completed and the results will be announced. This would make it impossible, in any case, to continue any diplomatic negotiations with the President of Russian Federation and he knows it very well," he went on.

"I think it's a very dangerous signal from President Putin that tells us that Putin is not going to finish this war. That is what's going on. "Again, they do not want any dialogue. We see referenda, we see mobilisation, that's emphasizing their will. They will not surrender. We need to keep applying pressure," he added.