The US has privately warned Russia for months against using nuclear weapons, telling Moscow that should it use them there will be consequences, according to a report published on Friday.
The State Department was involved in conveying the warnings, and the US may have also relied on sensitive intelligence channels to communicate them, according to CNN.
The report comes after Russian President Vladimir Putin issued a stark warning that he is prepared to use nuclear weapons as he and his proxies prepared to hold polls in separatist-controlled areas in Ukraine.
"When the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people. This is not a bluff," Putin said as he prepared to carry out referenda.
The polls, denounced as a "sham" by the US and its allies, began on Friday and are slated to run through Tuesday. In-person voting will only take place on the last day, Sept. 27, while the other days will see ballots being organized in communities and conducted door-to-door, according to local media reports.
Separatist-controlled Luhansk, as well as Donetsk, Kherson and Zaporizhzhia, which are partly under Russian control, announced plans for the referendums earlier this week.
Asked in remarks that aired on Sunday what he would do if Putin considered using nuclear weapons, US President Joe Biden said simply: "Don't. Don't. Don't."
"It would change the face of war unlike anything since World War II," he said. "Depending on the extent of what they do, will determine what response would occur."