A tank containing propylene, a colorless and odorless inflammable gas, exploded at approximately 10.00 a.m. Moscow time (07.00 a.m. GMT). The fire was extinguished half an hour later.
Three workers were also injured in the accident, and two are currently in intensive care with severe burns. A third worker has also been admitted to hospital, but is said to be in a stable condition.
The dead bodies are currently being identified.
"The explosion poses no health threat to local residents," an emergencies ministry spokesman said, adding that no local residents had sought medical help so far.
No leaks of poisonous substances, oil or oil products have so far been reported near the plant. However, eyewitnesses from a neighboring residential district have complained of headaches and breathing problems due to the thick smoke.
The petrochemical plant is a LUKoil subsidiary, and the company, Russia's largest independent oil producer, has said it will render all necessary assistance to the families of those killed and injured.
