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Viral videos are being pitched against each other in a battle for popularity under YouTube's new Slam service.

The Google-owned clip website has announced that it will put a new crop of videos "head-to-head" each week and ask users to vote for their favourite.

The firm described the offering as "a video discovery experiment".

Experts said the move was designed to keep visitors on YouTube for longer and deepen their ties to the website.

YouTube's blog said it was- comedy, cute, music, bizarre and dance.

Users' votes will determine where the clips end up on a leader board. Those taking part are promised "points for predicting the crowd favourites", allowing them to see how their predictive power compared to other players at the end of each week.

"You could help uncover the next big thing," said the firm's software engineer, Tomas Izo.

Music rival

Company watchers linked the move to the rise of Vevo - a music video website owned by Universal Music, Sony Music and the Abu Dhabi Media company.

The two-year-old service reported it had 57 million users in October. Although many of those viewers watched Vevo's clips through YouTube, increasing numbers streamed the videos direct through the Vevo's own mobile apps and website.

"YouTube is in a process of reinvention at the moment - Vevo is basically taking all of the music videos away from them," said Mark Mulligan, digital media analyst at the Music Industry blog.

"YouTube desperately need to create a new relevancy for themselves and that's why with the recent site relaunch - making it look more like a rich interactive TV-like experience - and features like Slam are all about trying to give YouTube a unique place in the online video landscape."

The Facebook effect

YouTube still ranks as the internet's most popular video website by a wide margin. A study by Comscore suggested that just under- accounting for roughly two in every five videos viewed online.

China's Youku came second, with 4.6 billion views. Vevo followed with 3.7 billion and Facebook with 2.6 billion.

Mr Mulligan believes that Slam was born out of YouTube's desire to deepen its ties with part of its audience, building on the kind of activity that many users have become accustomed to through social networks such as Facebook.

"For the vast majority of us we go to the site and watch a video or two and come out again. But for 5% to 7% of internet users, YouTube is almost like their main social destination - they have profile pages, they upload videos, they make comments, they like and dislike," he said.

"What Slam does is create something in the middle of those two groups.

"People have become more used to participating, commenting, liking and sharing. YouTube is trying to capitalise on that. Slam isn't for everyone or just for the 5-7%, it's something in the middle ground for the 25% sort of mark or so."