As if users needed another reason to spend more time on Facebook, Warner Bros has started offering films to rent, buy and view on the site.

The initiative, which was launched on Tuesday in a test phase, requires users to pay for the films using Facebook's on-site currency, called Credits.

Movies will be available on their individual Facebook Pages, starting with the latest Batman blockbuster, The Dark Knight, which costs $3 to rent.

More titles will be added over the coming months. The service is available only to US users.

With this move, Warner Bros adds a major digital distribution channel for its films, while Facebook enters the increasingly popular market for online streaming of professionally produced films and TV shows. This puts Facebook in competition with the likes of Apple, Amazon.com and Netflix.

In addition, letting its extremely hooked and loyal users watch full-length movies on the site is bound to boost Facebook's already robust usage metrics. According to comScore, in December Facebook unique visitors for the month grew 38 percent year-on-year to 153.9 million in the US, while total time spent on the site spiked 79 percent to almost 50 billion minutes.

It's not the first time that full-length, professional video is available on Facebook. In 2009, NBC debuted its show "Community" on the site and Hulu launched an application for watching NBC premieres.

"Right now, more than 400 games and applications use Facebook Credits to give people a convenient and safe way to buy virtual and digital goods on Facebook. We're open to developers and partners that want to experiment using Credits in new and interesting ways and we look forward to seeing what they come up with," a Facebook spokeswoman said via email when asked for comment about the Warner Bros announcement.