They breached security systems designed to keep out intruders by creating duplicates to 'SecurID' electronic keys from EMC Corp's RSA security division, an anonymous source told Reuters.

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.

They are the latest companies to be breached through sophisticated attacks that have pierced the defenses of huge corporations including Sony, Google Inc and EMC Corp.

Security experts say that it is virtually impossible for any company or government agency to build a security network that hackers will be unable to pierce.

The Pentagon, which has about 85,000 military personnel and civilians working on cybersecurity issues worldwide, said it also uses a limited number of the RSA security keys, but declined to say how many for security reasons.

The hackers learned how to copy those electronic keys with data stolen from RSA during a sophisticated attack that EMC disclosed in March, according to the source.

EMC declined to comment on the matter, as did executives at major defence contractors.

Lockheed, which employs 126,000 people worldwide and had $45.8 billion in revenue last year, said it does not discuss specific threats or responses as a matter of principle, but regularly took actions to counter threats and ensure security. Read More