"Unfortunately, at the moment, the officially confirmed death toll has increased to 17," Tver regional authorities said in a statement.

It added that efforts were under way to clear the debris.

A fire broke out last Thursday at the Central Research Institute of the Russian Air and Space Forces in Tver. The same day, local officials said that six people had been killed in the blaze.

Preliminary information suggested the fire was caused by faulty electric wiring.

There has been no official update on the cause of the fire.

According to the defence ministry, the institute focuses on research related to Russian air and space defence, as well as the development of new anti-aircraft systems.

Accidental fires are common in Russia, where hundreds of blazes are recorded each year due to ageing and dilapidated infrastructure and non-compliance with safety standards that are often lax.