Russia is expanding and renovating a Soviet-era laboratory that was once used for bioweapons research on the viruses that cause smallpox and Ebola.

Unusual activity was first captured at Sergiev Posad-6, north-east of Moscow, in satellite imagery a few months after Russia’s invasion of Ukraine in 2022.

Since then, ground has been broken on 10 new buildings across 250,000 square feet with signs they contain biological labs designed to handle extremely dangerous pathogens, The Washington Post reported.

The images showed layouts in keeping with partitioned laboratories, rooftop air handling units, heightened security features and what is thought to be a power plant.

It comes as Moscow’s full-scale invasion of Ukraine approaches its third year and reports mount that North Korean troops have been deployed to Russia in what the US and Nato have called a major escalation.