A judge on Thursday rejected a bid by Microsoft to derail a warrant demanding that email data from servers in Ireland be turned over to US prosecutors.

Microsoft vowed to battle on in the case, which is being closely watched by Internet firms eager to assure users around the world that their private information is not being freely shared with US authorities.

"The only issue that was certain this morning was that the District Court's decision would not represent the final step in this process," Microsoft general counsel Brad Smith said in an email reply to an AFP inquiry after the ruling by US District Judge Loretta Preska.

"We will appeal promptly and continue to advocate that people's email deserves strong privacy protection in the US and around the world."

Microsoft argued in court that the warrant, which would require the tech giant to turn over customer emails stored in a data center in Dublin, should be nullified because it would give the US government excessive power to pry over private information.

A two-hour hearing ended with Preska denying Microsoft's request to have the subpoena quashed, according to a spokesperson for the US attorney in New York.

- US snooping -

The legal battle comes amid rising concern about US surveillance following revelations of snooping disclosed by former National Security Agency contractor Edward Snowden. Leading tech firms, including Apple and Verizon, have filed briefs supporting Microsoft.

Microsoft has argued that the customer emails, sought in this case in a Justice Department narcotics probe, are entitled to the same protections as paper letters sent by mail.

That means prosecutors should only be able to access the information in the electronic "cloud" with a warrant, and that the authority of such warrants ends at the US border.

Smith also has publicly contended that the case could leave US citizen's privacy vulnerable to overseas prying if other counties opt for the same tactic.

But US Attorney Preet Bharara argued that under a 1986 law governing electronic communications, the tech giant is required to produce the data regardless of where Microsoft has decided to store it.

"Nothing in the text or structure of the statute carves out an exception for records stored abroad, and none exists in precedent," Bharara said in a court brief.

"Overseas records must be disclosed domestically when a valid subpoena, order or warrant compels their production."

US Magistrate Judge James Francis sided with the government, writing in an April decision that "it has long been the law that a subpoena requires the recipient to produce information in its possession... regardless of the location of that information."

Thursday's hearing focused on Microsoft's appeal of Francis's decision and took place in US District Court in downtown Manhattan.

The Snowden revelations have fueled efforts in some countries to require US tech firms to hold data within the country where it is generated, a move many firms say is impractical.

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MICROSOFT