Image copyright AFP Image caption The original Avatar was released in 2009 and took more than four years to make

The sequel to 2009 hit Avatar has been delayed a year and will not be released until 2017, James Cameron has revealed.

The Oscar-winning director of Aliens and Titanic said he is working on three new Avatar films at the same time.

The first film was supposed to be out in December 2016, but Cameron said writing the stories had become "very involved".

"There's a layer of complexity in getting the story to work as a saga across three films," he explained.

Cameron said the goal was to have the scripts for the three movies completed by the end of this month so that filming can begin.

"We're writing three simultaneously and we've done that so that everything tracks throughout the three films," he told the Associated Press.

Image copyright AP Image caption Cameron was speaking at an event in Wellington, New Zealand

"We're not just going to do one and then make up another one and another one after that.

"And parallel with that, we're doing all the design. So we've designed all the creatures and the environments."

Cameron said he thought it was important that each film linked forward to the next one in a satisfying way, but also came to a resolution that did not leave the audience hanging.

The three films will be shot in New Zealand, where some of the original film was also made.

Avatar is the highest-grossing film in history, with box office takings of almost $2.8 billion (£1.8 billion).

Set in the 22nd Century on a distant planet inhabited by a humanoid race, the sci-fi epic went on to win three Oscars.

Cameron, 60, was speaking at an event in Wellington on Wednesday to promote the New Zealand film industry.