Google's YouTube is set to launch its long-awaited monthly subscription service called YouTube Music Key in a few months, said Robert Kyncl, the online video service's head of content and business operations, at the Code/Media conference.

The beta version of the service is currently being tested and is available by invitation, Mr Kyncl said at the conference in California.

"There is a subset of audience who wants more things so they are used to paying for those. We don't think it changes anything for existing users," he said.

Google said in November the subscription for the ad-free service would start at $7.99 per month and allow users to watch videos offline and listen to music while other apps were working.

The plan would represent a significant change for the world's number one online video site, whose free videos, often accompanied by short commercials, attract more than 1 billion users a month.