The Pentagon has revealed that a defence industry computer network was the target of a cyber attack in March in which thousands of files were taken by foreign hackers.
Deputy Defence Secretary William Lynn said that in the attack and other breaches, hackers had taken information on ''our most sensitive systems''.
The BBC reports the Pentagon has rolled out a strategy for strengthening US cyber capabilities and addressing threats and attacks in cyberspace.
The plan would treat cyberspace in a similar manner to land, air and sea.
In a speech at National Defense University in Washington, Mr Lynn said about 24,000 files containing Pentagon data were stolen from a defence industry computer network in March.
He said at least one attack in 2008 which penetrated classified computer systems, came from a foreign intelligence service.
In an interview before the speech Mr Lynn said the Pentagon believed the March attack had been perpetrated by a foreign government.
The BBC reports previous cyber attacks have been blamed on China or Russia.