Police said the men, reported by Bulgarian media to be foreigners working on the film's second unit stunt team, were performing a stunt Thursday night involving an explosion in a rubber boat on the Ognyanovo dam near Elin Pelin, The Sofia Echo reported Friday.
The film's main unit was shooting several hours away in Bansko.
One man was killed at the site and the second man was taken to a hospital in Sofia with multiple injuries. He was said to be in stable condition, the English-language newspaper reported.
"It is with great regret that we confirm this unfortunate accident," Nu Image/Millennium Films 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 movie, which features actors Arnold Schwarzenegger, Sylvester Stallone, Bruce Willis, Jean-Claude Van Damme, Chuck Norris and Jason Statham, is set for release Aug. 17, 2012, The Hollywood Reporter said.