President Volodymyr Zelensky warned that the direct involvement of a third country “is the first step towards a world war”.

There is no evidence so far that biological weapons were used in Ukraine, which is prohibited by international treaties.

However, the development of the facility, as well as the secrecy surrounding it, has led to fears that it could be a possibility in the future.

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

US intelligence agencies and biological weapons experts are closely monitoring this development, The Post reported.

Russian forces are already uses chemical weapons on the front line 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 the two substances violates the Chemical Weapons Conventiona multilateral treaty which prohibits chemical weapons and of which Russia is a signatory.

Kremlin denies accusations

The Kremlin has in the past been accused of using nerve agents and illegal radioactive materials in high-profile assassinations against opposition figures and defectors, both at home and abroad.

Alexeï Navalny, key figure in the Russian opposition, was poisoned with Novichok planted in his underwear in Tomsk, Russia, 2020. Intelligence officer in exile Sergei Skripal was poisoned with the same substance in 2018 in Salisbury.

Baseless allegations that a US biological weapons program was being developed in Ukraine were one of the pretexts for its full-scale invasion, but the Kremlin denies the existence of biological weapons on its own soil.

A Soviet biological weapons network

The Sergiev Posad-6 complex was known as Zagorsk-6 during Soviet times and is believed to have been the center of military research into pathogenic viruses, particularly the variola virus that causes smallpox.

A vast Soviet network of biological weapons facilities working to weaponize diseases such as anthrax, smallpox and bubonic plague was exposed by Soviet defectors in the 1980s.

In 1979, a plume of anthrax spores was accidentally released from a secret facility near Yekaterinburg, killing 66 people in the deadliest anthrax outbreak ever known.

Russian officials also deny that the Soviet Union had an offensive biological weapons program.