The U.S. 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 U.S. officials.

It was not immediately clear how or when the warnings were sent. The State Department was involved, according to one official. The Biden administration has also leaned heavily on intelligence channels to communicate sensitive messages to Moscow throughout the buildup and prosecution of Russias war in Ukraine, including recently in the negotiations over wrongfully detained Americans.

The warnings, first reported by The Washington Post, come as Russian President Vladimir Putin has once again threatened to turn to nuclear weapons amid a series of embarrassing setbacks on the battlefield in Ukraine. In a speech on Wednesday, he warned that, In the event of a threat to the territorial integrity of our country and to defend Russia and our people, we will certainly make use of all weapon systems available to us. This is not a bluff.

U.S.officials have emphasized that this is not the first time Putin has threatened to turn to nuclear weapons since the start of his invasion of Ukraine in February, although some analysts have seen this threat as more specific and escalatory than the Russian presidents past rhetoric.

The U.S.has also sought to deter Russia from using a nuclear weapon in public warnings in the past and have made the issue a theme of remarks at the UN General Assembly this week in New York. Secretary of State Antony Blinken on Thursday said Russias reckless nuclear threats must stop immediately.

U.S.President Joe Biden, appearing on CBSs 60 Minutes last week, said his message to Putin should he be considering the use of nuclear weapons was: Dont. Dont. Dont.

The U.S.reaction would be consequential but would depend on the extent of what they do, Biden said, without providing further details.

For now, top CIA officials have said publicly that they have seen no signs that Russia is preparing to use nuclear weapons. But some military analysts have been concerned that Russia may seek to use a so-called tactical, or battlefield, nuclear weapon in response to its poor showing in Ukraine — a tactic sometimes called escalate to deescalate. Intelligence officials believe Putin would likely only turn to that option if he felt Russia or his regime were existentially endangered, and its not clear if he would feel that losing his war in Ukraine would fit that description.