In a single attack carried out in March foreign hackers stole over 24,000 data files from a compromised computer network, the US Department of Defense (DOD) has admitted.

The attack was part of a larger campaign by hackers against the DOD systems, and hackers are also trying to exploit its communications and satellite systems, according to William Lynn, the deputy secretary of defense. The most skilled hacking attempts are coming from nation states, rather than hacking groups, he warned.

"We have a pretty good idea who did it," Lynn told Associated Press in an interview, but refused to elaborate.

The news came as the US government has been outlining its online defence strategy. Currently the US is spending 90 per cent of its cybersecurity budget on defence and ten per cent of offence, but that ratio needed to be reversed warned Marine General James Cartwright, vice chairman of the Joint Chiefs of Staff.

"Going on the offensive is essential," said Wasim Ahmad, vice president at Voltage Security.

"In addition, traditional passive security systems that focus exclusively on protecting the IT infrastructure must be augmented with security that focuses on protecting the data itself. Even the Pentagon must assume that hackers will eventually get through to core data. And that data needs to be rendered useless to attackers by leveraging data encryption solutions."

Under the new DOD plans the online world will need to be addressed in the same way the DOD is responsible for land, air, water and space combat. The military will not rule out using physical force in response to hacking attacks, if they cause "massive damage, massive human losses, significant economic damage," Lynn said.

Stewart Baker, a former assistant secretary at the Department of Homeland Security, said that the plan had merit, but was lacking in key details.

"It's an incomplete description of a defensive strategy," he said.

"If the Pentagon announced that our nuclear warfare strategy centred on fallout shelters, we'd all hope that they had a real strategy that was better than that."