Harrison Ford has revealed that Star Wars: Episode VII is surrounded by mystery.

The Ender's Game star, who is rumoured to reprise his role as Han Solo in JJ Abrams' upcoming instalment, joked that he had been warned about saying anything about the film.

He remained tight-lipped about the stories, simply saying: "It's all shrouded in mystery."

Harrison - who stars as Colonel Hyrum Graff in Gavin Hood's big-screen adaptation of Orson Scott Card's sci-fi novel - said he enjoyed playing the commander of a military school, because he could boss his younger co-stars including Asa Butterfield and Hailee Steinfeld around.

"It was fun for me but these kids worked awful hard. The young people worked awfully hard to demonstrate the capacity of this zero gravity effect, and they had me barking at them when they weren't on camera," he said.

The 71-year-old said the role was one he couldn't turn down.

"For me, it was the opportunity to be part of something ambitious, creative and unique and to work with talented people all through from the writers and producers to the young and the older actors. It was just a great opportunity to me," he said.

"The character that I play and his relationship to Asa Butterfield is something I recognise as having real emotional opportunity, but also the experience of seeing that was something I thought might generate some questions in people's minds, which I thought was a good idea," he added.

Ender's Game, which also stars Sir Ben Kingsley and Viola Davis, opens in cinemas on October 25.