LOS ANGELES (AP) -- In an effort to kick-start still nascent sales of digital movies, 20th Century Fox has begun selling digital versions of the sci-fi thriller "Prometheus" three weeks ahead of the movie's release on Blu-ray and DVD.

The move is a reflection of the industry's effort to promote the concept of movie ownership on digital platforms.

Movie purchases from online retailers like Apple Inc.'s iTunes and Amazon.com are growing. But The Digital Entertainment Group, a movie-industry association, says such sales still represented less than 4 percent of U.S. home video spending in the first half of the year.

"Prometheus" is now on sale through iTunes, Amazon.com and Google's Play store for about $15.