A top Pentagon official has admitted that a massive amount of data related to new defence technologies were stolen earlier this year.

"It was 24,000 files, which is a lot, but I don't think it's the largest we've seen," William Lynn, the US deputy defence secretary, said on Thursday.

Lynn revealed the theft as he unveiled a new Pentagon cybersecurity strategy that designates cyberspace as an "operational domain", alongside sea, air and land, where US forces will practice, train and prepare to defend against attacks.

Critics say Pentagon cyber strategy does not have enough bite to counter breaches in defence networks

He said the theft occurred in March and targeted files at a defence contractor developing weapons systems and defence equipment.

However, he declined to specify the country behind the attack, which company was hit or what the files contained.

The hacking was a dramatic illustration of the rising difficulties the Pentagon faces in protecting military and defence-related networks critical to US security.

"The policy doesn't really deal with the legal justification for treating attacks on computer networks, public and private, as potential acts of war," Al Jazeera's Rossyln Jordan reported from the Pentagon.

Critics say the new strategy doesn't have enough bite to counter those types of breaches, much less the ones that could potentially cripple a nation.

Defence department employees operate more than 15,000 computer networks and seven million computers at hundreds of installations around the world. The department's networks are probed millions of times a day and penetrations have compromised huge amounts of data.

Lynn said a recent estimate pegged economic losses from theft of intellectual property and information from government and commercial computers at more than $1 trillion.

No group has come forward to claim credit for the breach referred to by Lynn. According to Lynn's comments, this particular hack is the work of a foreign intelligence service.

Cyberactivists linked to the Anonymous movement recently targeted contractors working for the US military establishment, including IRC Federal, an IT contractor, and Booz Allen Hamilton.

Source: Al Jazeera and agencies