The major US music labels joined the legal battle against file-sharing website Megaupload, with a copyright infringement lawsuit against the site shut down by US authorities.

The Recording Industry Association of America (RIAA) announced the civil lawsuit filed Thursday on behalf of Warner Music Group, Universal Music, Sony Music Entertainment and Capitol Records.

The latest action comes days after the US film industry filed a similar lawsuit seeking damages from what was claimed to be the biggest Internet site for online piracy before it was shut down by US authorities.

The RIAA suit says Megaupload and its founder Kim Dotcom "actively and intentionally encouraged their users" to upload illegal copies of "the most popular entertainment content, including plaintiffs' sound recordings, for the purpose of distributing those copies to millions of other users."

The complaint seeks the maximum allowable damages and also requests an injunction.

The latest suit comes as the US Justice Department and FBI are seeking to extradite Dotcom from New Zealand to face charges of racketeering fraud, money laundering and copyright theft in a US court, which could see him jailed for up to 20 years if convicted.

The earlier lawsuit was filed Monday by US film studios in coordination with the Motion Picture Association of America (MPAA).

Dotcom's lawyer Ira Rothken said on Twitter: "RIAA claims v Megaupload are meritless & is an assault on cloud storage a 'neutral' tech used both for good & bad."