The Pentagon may have to redesign some of its weapons system after a foreign intelligence service hacked into systems at a corporate contractor and obtained 24,000 key files in March.

The incursion was one of the worst single incidents the US defence department has seen. Though it did not name the contractor nor the country suspected of carrying out the attack, Lockheed Martin said in May that it had come under attack. China and Russia have frequently been suspected of carrying out internet espionage, with China the most prominent in recent years.

US defence chiefs now think they need to have a means of response against such incursions. "We're on a path that is too predictable, way too predictable," General James Cartwright, vice chairman of the joint chiefs of staff, said on Thursday. "It's purely defensive. There is no penalty for attacking us now. We have to figure out a way to change that." Hours later, the deputy defence secretary, William Lynn, presented a strategy whose thrust, he said, is defensive and focused on "denying the benefit of an attack".

Lynn revealed that over the year key files including plans for missile-tracking systems, satellite navigation, surveillance drones and even jet fighters have been stolen from systems. "A great deal of it concerns our most sensitive systems, including aircraft avionics, surveillance technologies, satellite communications systems and network security protocols," he said.

Attacks on defence-related contractors and systems are growing increasingly sophisticated. The hackers who broke into Lockheed Martin's systems had first raided the systems of EMC's security subsidiary RSA Security which provides cryptographic "keys" used to scramble and decode files, in order to gain remote access to staff computers.

Cartwright said US military commanders were now devoting 90% of attention to building better firewalls and only 10% to ways of deterring hackers from attacking. He said a better strategy would be the reverse, focusing almost entirely on attack.

The defence department's new strategy relies on deploying sensors, software and code to detect and stop intrusions before they affect operations. "If an attack will not have its intended effect, those who wish us harm will have less reason to target us through cyberspace in the first place," Lynn said. "Current countermeasures have not stopped this outflow of sensitive information. We need to do more to guard our digital storehouses of design innovation."

Cartwright suggested that stronger deterrents would be needed. "We are supposed to be offshore convincing people if they attack, it won't be free," he said, adding that adversaries should know that the US has "the capability and capacity to do something about it".

James Lewis, an expert on computer network warfare at the Center for Strategic and International Studies, told the New York Times the Pentagon's computer networks were vulnerable to security gaps in the systems of allies with whom the military cooperates. America's allies are "all over the map" on cybersecurity issues, Lewis said. "Some are very, very capable and some are clueless."

Lynn said most major efforts to penetrate crucial military computer networks were still undertaken by large rival nations. "US military power offers a strong deterrent against overtly destructive attacks," he said. "Although attribution in cyberspace can be difficult, the risk of discovery and response for a major nation is still too great to risk launching destructive attacks against the United States."

He warned that the technical expertise needed to carry out harmful internet raids was certain to migrate to smaller rogue states and to non-state actors, in particular terrorists.

If a terrorist group obtains "disruptive or destructive cybertools, we have to assume they will strike with little hesitation," Lynn said.

The Democrat congressman, Jim Langevin, co-founder of the congressional cyber security caucus, told the Washington Post the plan was a good start but that key areas were missing.

"What are acceptable red lines for actions in cyberspace?" Langevin asked. "Does data theft or disruption rise to the level of warfare, or do we have to see a physical event, such as an attack on our power grid, before we respond militarily?".

Lynn said the US has not yet been hit by an act of cyberwar and that there was deterrent value in remaining ambiguous about what would constitute one. But ultimately, he said, it is the president and the Congress that would decide that the human or economic damage is severe enough to consider a cyber event an act of war.