Marc Forster and Brad Pitt on the set of World War Z Credit: Flynet

Things are a little tense on the set of Brad Pitt's zombie thriller World War Z, to say the least.

According to Vulture, Pitt, 48, is no longer speaking with the film's director Marc Forster -- even though his production company, Plan B, lobbied hard for Forster to get the job. So when did the relationship turn sour?

Vulture sources cite a "combination of scheduling, script, and directorial problems," and say things have gotten so bad, Pitt and Forster will only communicate with one another via an intermediary.

Initially, the World War Z cast and crew were working hard -- and at an intense pace -- in order to put the film out by Christmas 2012. Pitt and studio execs weren't thrilled with how some of the movie's key set pieces were coming together, causing Paramount to push back its release to June 2013.

World War Z is also, currently, without a definitive ending, though when they brought in famed screenwriter Damon Lindelof to get his thoughts (a common practice among studios), the Lost producer said its lack of a conclusion was the least of the flick's problems.

Despite the shooting setbacks and interpersonal conflicts, some set insiders still insist all is not lost.

"The studio is cultivating multiple options," one source explained. "One is to try scrapping [the ending] and trying something different: They want to construct an entirely new ending. The other is to try salvaging it . . . This is not an unmitigated disaster; it is salvageable."

Vulture notes they reached out to reps for Pitt and Forster for comment; both declined to elaborate on the latest with World War Z.