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 those sales still represented less than 4 percent of U.S. home video spending in the first half of the year, according to The Digital Entertainment Group, a movie-industry association.

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

Fox said it will push digital movie sales this way in a campaign it is calling "Digital HD." For future releases, it plans to give high-definition digital sales a two-week head start before the disc release.

The digital versions of Fox titles don't come with all the extras included with Blu-ray copies, such as deleted scenes, and they are not as high-quality. The iTunes version of "Prometheus" in HD offers viewing in 720p, a standard which is not as sharp as the 1080p offered on Blu-ray. An "SD," or standard definition version, was available from iTunes for pre-order for $13.