The United States for several months has been sending private communications to Moscow warning Russia’s leadership of the grave consequences that would follow the use of a nuclear weapon, according to U.S. officials, who said the messages underscore what President Biden and his aides have articulated publicly.

The Biden administration generally has decided to keep warnings about the consequences of a nuclear strike deliberately vague, so the Kremlin worries about how Washington might respond, the officials said, speaking on the condition of anonymity to describe sensitive deliberations.

The attempt by the White House to cultivate what’s known in the nuclear deterrence world as “strategic ambiguity” comes as Russia continues to escalate its rhetoric about possible nuclear weapons use amid a domestic mobilization aimed at stanching Russian military losses in eastern Ukraine.

The State Department has been involved in the private communications with Moscow, but officials would not say who delivered the messages or the scope of their content. It was not clear whether the United States had sent any new private messages in the hours since Russian President Vladimir Putin issued his latest veiled nuclear threat during a speech announcing a partial mobilization early Wednesday, but a senior U.S. official said the communication has been happening consistently over recent months.