SAN FRANCISCO, March 6 (AFP) - A federal court Tuesday ordered the popular music-swapping website Napster to block access to copyrighted music sent over its computers.

But US District Court Judge Marilyn Patel also ruled that music producers share the burden of identifying copyrighted songs transmitted through Napster.

Under Patel's preliminary injunction, once Napster is notified about a copyright violation using its computers, it must block the transmission within three business days. Napster was also given five business days to show it is complying with the order.

Analysts were divided over the ruling's impact: some called it a fatal blow to one of the Internet's fastest-growing sites, while others said Napster could still operate under the order.

The case is seen as a key test of whether copyright and other intellectual property laws can be applied in cyberspace, where music, movies and other forms of entertainment can be transferred with the click of a mouse.

Napster has attracted more than 60 million users worldwide in less than two years, but the recording industry and others contend it allows massive piracy of copyrighted works.

After an appeals court reaffirmed that the site violates copyright laws, Napster and the recording industry clashed in a hearing last week over whose responsibility it was to identify protected songs.

Patel's ruling, dated Monday and posted on the court's website Tuesday, indicated both sides share the responsibility for identifying copyrighted content.

"All parties shall use reasonable measures in identifying variations of the filename(s), or of the spelling of the titles or artists names, of the works identified by plaintiffs," the judge wrote.

Patel conceded it might be "difficult" for record companies to identify all the copyrighted files that can be downloaded using Napster, but added "this difficulty, however, does not relieve Napster of its duty."

The recording industry reacted favorably to the decision.

"We are gratified the District Court acted so promptly in issuing its injunction," said Hilary Rosen, president of the Recording Industry Association of America, one of the plaintiffs in the case.

"We intend to provide the notifications prescribed by the court expeditiously, and look forward to the end of Napster's infringing activity."

Napster officials could not immediately be reached for comment.

Nicholas Economides , an economics professor at New York University, said he sees the decision as "the beginning of the end for Napster, at least for Napster as a non-subscription service."

Economides said the restrictions on Napster will likely drive digital music fans to other systems that are free and use no central server.

"People will go the public domain programs like Gnutella or Freenet," he said. "They are not commercial entities, they don't have servers, they cannot be sued."

But Malcolm Maclachlan, Internet media analyst with IDC, said Napster can operate as long as it makes good faith efforts to comply with the court order.

"Napster doesnt need to play hide and seek because its users are playing hide and seek," Maclachlan said, noting that users can continually change the names of files being transferred.

Mark Lewis of the digital entertainment firm Webnoize said both sides now face challenges.

The music industry has given Napster a list of some 5,600 songs, he noted, "but they will have to deliver much more extensive lists before they can effectively force Napster users to halt the enormous distribution of copyrighted music."

Webnoize estimated that Napster users downloaded 2.97 billion songs in February, an average of some 100 million per day.

Lewis said that even though Napster began blocking some song titles in advance of the injunction, "the file-filtering will have to be stepped up to comply fully with the injunction. This will be a daunting task."

The recording industry also faces the task of proving the copyright of all the songs on Napster.

"In some cases, these certifications will be difficult to retrieve from archives," Lewis said. "Moreover, it is possible that Napster could challenge the validity of some certifications."

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