Deputy defense secretary William J. Lynn revealed in a speech on Thursday that the security systems of a defense contractor were breached by hackers back in March, resulting in the loss of 24,000 military files. The hackers are believed to be based outside of the US, though no specific information as to their whereabouts was given.

According to a Huffington Post report, the cyberattack is one of the Pentagon’s biggest ever security breaches.

Lynn called it a “significant concern” that “over the past decade, terabytes of data have been extracted by foreign intruders from corporate networks of defense companies.”

The deputy defense secretary went on to explain that while much of the data taken by intruders is “mundane,” some of it is extremely sensitive, relating to areas such as aircraft, surveillance and satellite communications.

Lynn was outlining the Pentagon’s new cyber strategy, which aims to produce tougher computer networks that can withstand cyberattacks and allow the military to continue to function in an effective manner, even when parts of its set-up have been compromised.

It is the first time such a strategy has been implemented by the Pentagon as it looks at ways to deal with cyberattacks not only from foreign groups and governments looking to obtain sensitive data, but also terrorist groups.

“Current countermeasures have not stopped this outflow of sensitive information,” Lynn also said in his speech, adding: “We need to do more to guard our digital storehouses of design innovation.”

Last month, national security expert Richard Clarke claimed that the Chinese government is systematically attacking the computer networks of the U.S. government.

In late May, hackers were also reported to have gained access to the computer networks of defense contractors Lockheed Martin.

In the same month, White House officials were the victims of an attempt by foreign hackers to trick them into giving over their e-mail passwords. Military personnel were among those targeted. China was blamed for the phishing attack, though the government denied being involved.

Also in May, the Pentagon announced that cyberattacks on US computer systems could be regarded as an act of war, resulting in a possible military response.