"On six occasions in recent weeks we agreed with the department of justice to extend the government's deadline to reply to these lawsuits. We hoped that these discussions would lead to an agreement acceptable to all," Smith wrote.

The negotiations failed, he wrote, so Google and Microsoft were going to court. He did not specify when, or to which court.

Snowden, a former NSA whistleblower, gave documents to the Guardian and Washington Post revealing NSA claims about access to technology firms' data via its Prism system.

The companies denied the NSA had "direct access" to their systems but said they were legally unable to disclose how many times they have been asked to provide information on users.

Fisa requests are granted by a special court that sits in secret and can grant the NSA permission to collect data stored by any company about a named person. In 2012, the court granted 1,856 requests and turned none down.

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