Lockheed Martin Corp, the US government's top information technology provider, said it detected and thwarted "a significant and tenacious attack" on its information systems network one week ago.

"As a result of the swift and deliberate actions taken to protect the network and increase IT security, our systems remain secure," Jennifer Whitlow, a Lockheed spokeswoman, said in an e-mailed statement.

"No customer, program or employee personal data has been compromised."    

Lockheed's information security personnel are working around the clock to restore employee access to the "information systems network" targeted in the May 21 attack, the statement said.

Bethesda, Maryland-based Lockheed, the Pentagon's No. 1 supplier by sales and the world's largest aerospace company, has kept the "appropriate US government agencies" informed of its actions, it added.

The Department of Homeland Security said it and the Defense Department had offered to help gauge the scope of a "cyber incident impacting LMCO," as the maker of fighter jets, ships and other major weapons systems is known.

The US government also has offered to help analyse "available data in order to provide recommendations to mitigate further risk," Chris Ortman, a Homeland Security official, said in an e-mailed reply to a query from Reuters.

A person with direct knowledge told Reuters on Friday that unknown attackers had broken into sensitive networks of Lockheed Martin and several other US military contractors.   

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 authorised to discuss the matter publicly. - Reuters