There is no evidence biological weapons have been used in Ukraine so far, something outlawed under international treaties.

However, the development of the facility, as well as the secrecy surrounding it, has raised concerns it could be a possibility in the future.

Russian officials have in recent weeks confirmed publicly that the Sergiev Posad-6 labs will be used to study deadly microbes, something they said was necessary to defend the country against bioterrorism and future pandemics.

US intelligence agencies and bioweapons experts are monitoring the development closely, The Post reported.

Russian forces are already using chemical weapons on the frontlines in Ukraine. In May, the US state department said it had evidence of the use of chloropicrin, a powerful irritant more toxic than chlorine that damages the lungs, as well as riot control agents.

The use of both substances violates the Chemical Weapons Convention, a multilateral treaty that bans chemical weapons and to which Russia is a signatory.