The Recording Industry Association of America has accused Napster of "failing to abide by either the letter or the spirit" of the injunction placed against it by the US Courts.

The music business group is accusing Napster of having deliberately shirked its obligations under the injunction. The RIAA attacks the filters Napster has put in place to block copyrighted material from its service, its efforts to block files, and countermands claims made by Napster in a compliance brief the company filed last week.

Napster's interim chief executive officer (CEO) Hank Barry responded forcefully, saying: "Napster is aggressively complying with the injunction with significant measurable results."

Non-cooperation The RIAA's call for a change in the filtering technology Napster uses, according to Barry, is "an attempt to change the subject rather than cooperate with Napster as the injunction specifies".

Napster was originally sued for copyright infringement in late 1999 by the RIAA.

RIAA claims it and its member record companies have undertaken a "gargantuan effort" - 1,800 man-hours by the record labels and an additional 600 hours by the RIAA - to provide Napster with 8,001,913 filenames corresponding to 328,074 works to be blocked.

However, all the songs on a list of 675,000 - included whether or not they were found on the system and given to Napster with the demand they be blocked - are still available on the network. As are more than 70 per cent of the songs Napster claims to have blocked, said the RIAA.

Filter fight The failure of the filters is due only in part to the technology that was chosen, the brief said. The RIAA added: "It is doubtful that Napster's self-selected, technologically archaic filter could ever significantly limit access to plaintiff's music. Yet, Napster contemptuously refuses to employ an effective filter - for fear that it might actually work."

The RIAA says that the company ought to base its exclusions of files on their checksums. A checksum is a unique, mathematically generated "fingerprint" contained in every MP3 that was created from the same CD with the same software. For example, if a user were to create an MP3 of Britney Spears' (pictured) "Oops, I Did it Again", every copy based on that original file would contain the same checksum. Such a system would not add substantial burden to Napster, the RIAA contends, because the Napster software already relies on checksums and includes them in its database.

On Tuesday, Napster's Barry said: "In the three weeks since the court's injunction was issued, Napster has blocked access to over 275,000 unique songs and over 1.6 million unique filenames." Napster also has added more than 10,000 variations in artists' names and more than 40,000 variations in song titles, the statement said.

Napster welcomes any technology to block copyrighted works, but believes the technical issues should be decided by an independent arbiter rather than by the RIAA, Barry said.