Documents obtained by the Electronic Frontier Foundation (EFF) claim to show that US telco AT&T allowed the National Security Agency (NSA) to set up a 'secret room' in its offices to monitor internet traffic.

The documents were handed over as part of an EFF legal case against AT&T for alleged violation of user privacy. The US government has asked the courts to dismiss the case, claiming that the lawsuit could expose state secrets.

"The district court rejected the government's attempt to sweep this case under the rug," said EFF senior staff attorney Kurt Opsahl.

"This country has a long tradition of open court proceedings, and we are pleased that, as we present our case to the Court of Appeals, the millions of affected AT&T customers will be able to see our arguments and evidence and judge for themselves."

The room, described as secret and secure, houses surveillance equipment used to spy on AT&T customers. Investigations could include web use, email and voice communications.

"This is critical evidence supporting our claim that AT&T is cooperating with the NSA in the illegal dragnet surveillance of millions of ordinary Americans," said EFF legal director Cindy Cohn.

"This surveillance is under debate in Congress and across the nation, as well as in the courts.

"The public has a right to see these important documents, the declarations from our witnesses and our legal arguments, and we are very pleased to release them."