Six people died on Thursday when a fire engulfed a Russian military research institute involved in the development of air defense systems in the northwestern city of Tver, local officials said.
Russian television showed thick black smoke rising from inside the four-story yellow building of the Central Research Institute of the Russian Air and Space Forces.
Several employees of the institute had to jump from the windows of the upper floors to escape the flames, according to witnesses cited by the broadcasters.
The Tver regional government said in a statement carried by state media that six people were killed and 21 injured in the fire, updating previous tolls.
Preliminary information suggests the fire was caused by faulty electrical wiring, state news agency TASS said.
The Defense Ministry said the institute focuses on research related to Russian air and space defense, as well as the development of new anti-aircraft systems.
Accidental fires are common in Russia, where hundreds of fires are recorded each year due to aging and dilapidated infrastructure and failure to comply with often lax safety standards.
AFP contributed reporting.