Unknown hackers have broken into the security networks of Lockheed Martin and several other US military contractors, a source with direct knowledge of the attacks told Reuters.

They breached security systems designed to keep out intruders by creating duplicates to "SecurID" electronic keys from EMC Corp's RSA security division, said the person who was not authorized to publicly discuss the matter.

It was not immediately clear what kind of data, if any, was stolen by the hackers.

But Lockheed's and other military contractor networks house sensitive data on future weapons systems as well as military technology currently used in battles in Iraq and Afghanistan.-Reuters