A stuntman has died and another has been seriously injured on the set of Sylvester Stallone's film sequel The Expendables 2 in Bulgaria. The stuntman, who has not been named, died while performing a stunt which involved an explosion. The second injured man is reported to have undergone surgery and is in a critical condition in hospital. Police have launched an investigation into Thursday's accident which happened 15 miles outside the capital Sofia. "It is with great regret that we confirm this unfortunate accident," Nu Image/Millennium Films, the production company making The Expendables 2, said in a statement. "Our hearts go out to the families and those on the production affected by this tragedy. "The filmmakers are working closely with the authorities in responding to and investigating this accident." The accident happened during filming of an explosion on an inflatable boat on the Ognyanovo dam. Stallone, who directed the first Expendables film and is producing the sequel, was not present at the time of the accident as he was filming other scenes in another location. The Expendables 2 also features a star-studded action hero cast that includes Stallone, Arnold Schwarzenegger, Bruce Willis, Dolph Lundgren, Jet Li, Chuck Norris and Jean-Claude Van Damme.