STORY: The source, who asked to remain unidentified, said the talks are ongoing. The Sullivan talks were first reported by the Wall Street Journal.
Sullivan said in September that the United States had communicated publicly and privately with the Russians about Russian President Vladimir Putin's threats to use nuclear weapons in the Ukraine war.
While Sullivan had said publicly that there would be tragic consequences should Russia resort to nuclear weapons, it was unclear how this message was being communicated privately. The source said the talks had been taking place in recent months.