UPDATE 1-Lockheed network suffers major disruption - sources

* Lockheed declines to discuss any specific threat

* Says policies in place to ensure security (Adds further detail, comments by technology experts)

By Andrea Shalal-Esa and Jim Finkle

WASHINGTON/BOSTON, May 26 Lockheed Martin Corp (LMT.N), the Pentagon's No. 1 supplier, is experiencing a major disruption to its computer systems that could be related to a problem with network security, two sources familiar with the issue said on Thursday.

Lockheed, the biggest provider of information technology to the U.S. government, is grappling with "major internal computer network problems," said one of the sources who was not authorized to publicly discuss the matter.

A second source, who also asked not to be identified, said the issue was "affecting a lot of people" at Lockheed, maker of the stealthy F-22 and F-35 fighter planes and other critical weapons systems.

Lockheed, which employs 133,000 people worldwide and had $45.8 billion in revenues last year, said it did not discuss specific threats or responses as a matter of principle, but regularly took actions to counter threats and ensure security.

"We have policies and procedures in place to mitigate the cyber threats to our business, and we remain confident in the integrity of our robust, multilayered information systems security," said spokesman Jeffery Adams.

Big corporations -- especially government contractors -- keep matters of internal security secret and rarely publicly disclose problems in securing their networks.

The sources said Lockheed employees were still able to use mobile devices to access their company email accounts.

The slowdown began on Sunday after security experts for the company detected an intrusion to the network, according to technology blogger Robert Cringely. He said it involved the use of SecurID tokens that employees use to access Lockheed's internal network from outside its firewall,

A spokesman for EMC Corp EMC.N, whose RSA division makes the tokens, declined to comment, saying it is company policy to never discuss security issues affecting specific customers.

EMC disclosed in March that hackers had broken into its network and stolen some information related to its SecurIDs. It said that the information could potentially be used to reduce the effectiveness of those devices in securing a customer's network.

Steve Winterfeld, cyber technical lead at TASC, an advanced systems company spun off from Northrop Grumman Corp (NOC.N), said RSA had not provided any details of how hackers broke into its network, which meant the hackers could have used the same method to attack other corporations.

He said TASC and other companies were extremely concerned about the breach, which meant that the SecurID tokens could no longer be viewed as completely secure.

"You have no idea how many people are freaked out right now," Winterfeld told Reuters. "TASC is no longer treating the RSA device as if it were as secure as it was beforehand." (Reporting by Jim Finkle and Andrea Shalal-Esa; Editing by Tim Dobbyn)