Hackers have forced the Pentagon to shut down a large number of computers by penetrating parts of the security system.

Reports suggested that 1,500 computers were taken offline, but the US Department of Defence said it could not confirm how many computers were affected.

Navy Lieutenant Commander Chito Peppler, a Pentagon spokesman, said that some computers were still out of action after the attack, which happened on Wednesday. He said that they would be back to normal soon.

Robert Gates, the US Defence Secretary, said that the computers were shut down when a breach of the system was detected. He said the cause was being investigated.

Few details about the attack were released, but Donal Casey, a security consultant at Morse, said that the action taken by the Pentagon suggested that the breach was serious.

“The most frequent form of attack is denial of service attacks [in which hackers swamp a website by directing thousands of computers to access it simultaneously], but they can usually be handled without taking systems down,” Mr Morse said. “In this case it seems that there was an actual breach and someone was on the system.”

Lieutenant Commander Peppler said that Defence Department systems are probed every day by a wide variety of attacks.

“The nature of the threat is large and diverse, and includes recreational hackers, self-styled cyber-vigilantes, various groups with nationalistic or ideological agendas, transnational actors and nation-states,” he said.

Mr Gates said that Pentagon operations were not affected, but acknowledged that there would be “some administrative disruptions and personal inconveniences.” When asked whether he was personally affected he said: “I don’t do e-mail. I’m a very low-tech person.”

Although security for Pentagon networks is among the tightest in the world, the threat could not be completely eliminated, Mr Casey said. “By the nature of websites and e-mail systems that are public-facing, they’re always exposed,” he said.