(CNN) — Secretary of Defense Lloyd Austin in a new interview condemned “nuclear saber-rattling,” and said while he hasn’t seen anything to suggest Vladimir Putin has decided to use nuclear weapons in the ongoing war on Ukraine, the choice is up to the Russian President.

“To be clear, the guy who makes that decision, I mean, it’s one man,” Austin said of Russian threats of nuclear weapons in an interview with CNN’s Fareed Zakaria, set to air in full Sunday on “Fareed Zakaria GPS.”

“There are no checks on Mr. Putin. Just as he made the irresponsible decision to invade Ukraine, you know, he could make another decision. But I don’t see anything right now that would lead me to believe that he has made such a decision.”

Austin told Zakaria that he has privately conveyed to his Russian counterpart, Russian Minister of Defense Sergei Shoigu, not to “go down this path and conduct this type of irresponsible behavior.” Austin said he has not talked to Shoigo in “recent days” but said other members of US government leadership have conveyed similar messages to Russia “recently.”

“You’ve heard people in our — in our leadership — among our leadership that have said that we have communicated to them recently,” Austin said. “Personally, I have not talked to Shoigu in recent days, but I have talked to him in the past. And I have addressed this very issue to — and warned to not go down this path and conduct this type of irresponsible behavior.”

As CNN previously reported, the US has privately communicated to Russia for the past several months that there will be consequences if Moscow chooses to use a nuclear weapon in the Ukraine war, according to US officials. It was not immediately clear how or when the warnings were sent. The State Department was involved, according to one official.

Austin said Putin’s annexation claims to Ukrainian territory were “illegal” and called threats to use nuclear weapons “an irresponsible statement.”

“This nuclear saber rattling is not the kind of thing that we would expect to hear from leaders of large countries with capability,” Austin said.

Putin on Friday announced Russia would seize nearly a fifth of Ukraine, declaring that the millions of people living there would be Russian citizens “forever,” following so-called referendums, which have universally been dismissed as shams by Ukraine and Western countries.

US President Joe Biden and Secretary of State Antony Blinken condemned the annexation and announced new sanctions against Russia in response.

Austin said the US will continue to support Ukraine in its efforts to take back the territory that has been captured by Russia.

“We can expect that the Ukrainians will continue to move forward and attempt to take back all of the territory within their — within their sovereign borders here,” he said. “We will continue to support them in their efforts.”