Washington (AFP) - The Democratic National Committee has agreed to restore access to a key voter database to the campaign of US presidential hopeful Bernie Sanders following uproar over a breach.

The DNC had on Thursday temporarily suspended the campaign's access to the database after at least one of its staffers took advantage of a computer glitch to peek at crucial voter data for rival Hillary Clinton.

The suspension was a slap to Sanders' hopes of scoring an upset over Clinton for the Democratic nomination because the database features information on voters nationwide, and candidates use it to plan strategy as they vie to replace Barack Obama.

DNC Chair Debbie Wasserman Schultz said access was being restored after the campaign complied with a request for information, but said an investigation would continue to ensure the data has been erased.

"The fact that data was accessed inappropriately is completely unacceptable," she said in a statement early Saturday.

As tensions mounted over the matter, the Sanders team filed a 12-page lawsuit with the US Court for the District of Columbia against the DNC over breach of contract on Friday.

The Vermont senator's campaign also fired one staffer and the party blamed NGP VAN, the technology company that provides campaigns with access to the data, for allowing the intrusion.

In its statement on renewed access, the Sanders campaign claimed the DNC had "capitulated" and said its "about face" came as a deadline approached for a hearing on a motion for an emergency injuction.

"Clearly, they were very concerned about their prospects in court," said Jeff Weaver, Sanders' campaign manager.

The issue is sure to surface Saturday evening when Clinton, Sanders and former Maryland governor Martin O'Malley face off in the party's third presidential debate in the early voting state of New Hampshire.

- Not the first glitch -

Sanders, an independent who calls himself a democratic socialist, is running a distant second in national polls behind Clinton, who has a more than 20 point lead in the race for the Democratic presidential nomination.

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The glitch -- in which a firewall protecting proprietary data was lowered briefly on Wednesday -- made voter data unique to the Clinton campaign viewable for outside eyes.

The dismissed staffer, Josh Uretsky, told CNN he had not been trying to spy on Clinton.

Rather, Uretsky said he had become aware of the breach and just poked around in the Clinton data for a while to see how exposed the Sanders campaign was. Then the firewall protecting the data went back up.

"To the best of my knowledge, nobody took anything that would have given the campaign any benefit," Uretsky told CNN.

Although Uretsky was the only staffer fired, The New York Times reported that during the time that Clinton's data was exposed, searches were carried out by four user accounts associated with the Sanders campaign.

Sanders' campaign spokesman Michael Briggs said it was not the first time the technology company has dropped the firewall.

He said the Sanders team had learned of such glitches months ago and told the DNC that campaign data, including Sanders material, was being made visible to other campaigns.

"Unfortunately, yesterday, the vendor once again dropped the firewall between the campaigns for some data," Briggs said late Thursday.

"After discussion with the DNC, it became clear that one of our staffers accessed some modeling data from another campaign. That behavior is unacceptable and that staffer was immediately fired."