WASHINGTON - The Pentagon suffered one of its largest-ever cyber thefts this spring when more than 24,000 files were stolen by a foreign government, officials disclosed on Thursday. William Lynn, the deputy secretary of defence, said at the National Defence University in Washington that the files were stolen from a defence industry computer in a single intrusion in March. He did not say who was believed to be behind the attack or describe the nature of the files that were stolen. It is a significant concern that over the past decade, terabytes of data have been extracted by foreign intruders from corporate networks of defence companies, he said at the start of an afternoon speech laying out the Defence Departments first unified strategy for cyber security. Indeed, in a single intrusion this March, 24,000 files were taken. Lynn said the massive attack was not by an individual but by another country. It was done, we think, by a foreign intelligence service, he said, declining to identify the country. Theft was data-related, he said. That cyber break-in and others have galvanized the Pentagon to develop new cybersecurity rules aimed at guarding against attacks coming from within the military and outside it. It is critical to strengthen our cyber capabilities to address the cyber threats were facing, Defence Secretary Leon Panetta said in a statement ahead of Lynns speech. I view this as an area in which were going to confront increasing threats in the future and think we have to be better prepared to deal with the growing cyber challenges that will face the nation. Lynn called some of the data stolen in cyberattacks mundane, but added, A great deal of it concerns our most sensitive systems, including aircraft avionics, surveillance technologies, satellite communications systems, and network security protocols. And websites and computer systems throughout across the defence world have been hit, he warned. In fact, our venue here today, the National Defence University, has been struck, he said. The NDU website and its associated server were recently compromised by an intrusion that turned over system control to an unknown server. The Pentagon is declaring the virtual world to be a new warfare domain like the land, sea and air. The military, the plan says, must continue to operate even if its computer systems are attacked. The plan includes new rules that stress deeper defences, greater collaboration between the Pentagon and the defence industry, and measures to prevent theft by malicious insiders. Were on the bad side of a convergent threat, Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, said earlier Thursday. Weve got to change that around and part of that will be the deterrent construct. Right now were on a path thats predictable, way too predictable, he said.