Belarusian President Aleksander Lukashenko says Russian tactical nuclear weapons are being transported to his country.
It comes after he signed an agreement earlier this week to formalize the deployment.
Lukashenko said, "We had to get ready and prepare storage areas. We managed to do that, so the transfer of nuclear weapons has already started."
Russian President Vladimir Putin announced plans to deploy tactical nuclear weapons to Belarus in March.
He said Russia planned to finish building a dedicated storage facility by July 1.
The deployment is widely seen as a warning to the Western nations that are ramping up military support for Ukraine.
The United States strongly condemned the move. State Department spokesperson Matthew Miller said, "As we have made clear, the use of chemical, biological or nuclear weapons in this conflict would be met with severe consequences."
But the Institute for the Study of War cast doubt on Lukashenko's comments in an assessment published Thursday. It said, "Russia has not yet deployed nuclear weapons to Belarus and their possible deployment is highly unlikely to presage any Russian escalation."
The US-based think tank added that Russia likely intends to use the deployment to "further subordinate the Belarusian security sphere under Russia."