Sigourney Weaver has signed on to return for the three upcoming Avatar sequels as a new "challenging character".

The Alien star played Grace Augustine in the first film, but director James Cameron was so eager to reunite with Weaver that he has created a new role for her in the follow-ups.

The Titanic director says in a statement which reads: "Sigourney and I have a long creative history, dating back to 1985 when we made Alien. We're good friends who've always worked well together, so it just feels right that she's coming back for the Avatar sequels.

"Her character of Grace Augustine, as fans know, died in the first movie, so she's playing a different and in many ways more challenging character in the upcoming films. We're both looking forward to this new creative challenge, the latest chapter in our long and continuing collaboration."

Weaver joins returning cast members Zoe Saldana and Sam Worthington for the sequels, with the first of three scheduled to hit cinemas in December, 2016.