The US has been privately warning Russia against using nuclear weapons in Ukraine, per The Washington Post.
They had been kept purposefully vague so that the Kremlin worries about how the US might respond.
Putin recently threatened to use nuclear weapons in an escalation of rhetoric over his war in Ukraine.
The Biden administration has been privately warning Russia against using nuclear weapons in Ukraine for months, the Washington Post reported, citing US officials.
The warnings have been kept purposefully vague so that the Kremlin worries about how Washington might respond, the officials speaking on the condition of anonymity said.
Officials would not specify the content of the private messages or reveal who delivered them but said the State Department had been involved.
In a rare address to the nation last week, Russian President Vladimir Putin threatened to use nuclear weapons in an escalation of rhetoric over his war in Ukraine.
It was not clear whether the Biden administration sent any new private messages following the threat, The Washington Post reported.
The private warnings echo the public statements made by President Joe Biden and his aides, which follow a nuclear deterrence world as "strategic ambiguity," the report said.
Experts and the international community are divided about how seriously to take Putin's nuclear threats.
Top White House official John Kirby told the BBC on Friday that the US was taking Putin's threats "seriously."
"It is a dangerous precedent for Mr. Putin to be using this kind of rhetoric in the context of a war clearly that he's losing inside Ukraine," Kirby, who is the National Security Council spokesperson, told the outlet.
"We have to take these threats seriously, and we do. We've been monitoring, as best we can, his nuclear capabilities, I can tell you that we don't see any indication that we need to change our strategic deterrent posture at this point."
However, former NATO commander, Adm. James Stavridis, told MSNBC that he was not concerned, saying that Putin was "upping the ante" but was unlikely to actually use nuclear weapons.
Other analysts have made similar assessments, with Russia expert Simon Miles recently telling Insider that Putin's threats are an indication that the president is increasingly aware of how limited his military options are in Ukraine.
Biden administration officials have noted that Russia has made threats about nuclear weapons since it began its invasion of Ukraine in February and that there are no indications Russia is moving nuclear weapons in preparation for a strike, per The Washington Post.
However, Putin's recent threat is more overt than previous ones.
The Russian president's latest comments suggest that Russia is considering using a nuclear weapon on the battlefield in Ukraine, Daryl Kimball, executive director of the Arms Control Association, a nonproliferation advocacy group in Washington told the paper.
"What everyone needs to recognize is that this is one of, if not the most, severe episodes in which nuclear weapons might be used in decades," Kimball told The Post.
"The consequences of even a so-called 'limited nuclear war' would be absolutely catastrophic."