Joss Whedon has promised there will be a big twist in the 'Avengers Assemble' sequel.

The director will be in charge of the follow-up to the hugely successful superhero ensemble movie and while he refuses to give away plotlines, he says fans won't be disappointed.

He said: ''The creation of the team is not the happy ending. It is the beginning of something that is complex and difficult, and now I get to dig a little deeper. And maybe, while I'm digging, just twist that knife. And that's exciting for me.''

The 'Avengers Assemble' sequel will be released on May 1, 2015, but a number of cast members will be unavailable to shoot the movie ready for an earlier release date as they will be filming their own standalone motion pictures.

Robert Downey Jr. is to reprise his titular role in 'Iron Man 3' in 2013, Chris Evans will return as the lead character in 'Captain America: The Winter Soldier' in 2014 and Chris Hemsworth is to star in 'Thor: The Dark World' next year.

A source said: ''In terms of the time frame, I think that it is well in the works. It looks like the release date might be May 1, 2015, but that can change depending on people's schedules.

''The beauty of 'Avengers Assemble' is that you've got so many people in this that even during the shoot if someone has to do another film they can just focus on [other characters].''