Lockheed Martin Corp, the US government's top information technology provider, said on Saturday it had thwarted "a significant and tenacious attack" on its information systems network a week ago but was still working to restore employee access. No customer, program or employee personal data was compromised thanks to "almost immediate" protective action taken after the attack was detected May 21, Jennifer Whitlow, a company spokeswoman, said in an e-mailed statement. She said the company, the world's biggest aerospace company and the Pentagon's No 1 supplier by sales, was working around the clock to restore employee access to the targeted network while maintaining the highest security level. The US Defence Department said in statement late Saturday night that it was working with Lockheed to determine the scope of the attack. The incident's impact on the department is "minimal and we don't expect any adverse effect," Air Force Lieutenant Colonel April Cunningham said by email. She declined to specify the nature of the impact, saying that as a matter of policy, the department does not not comment on operational matters. The Department of Homeland Security, or DHS, said that it and the Defence Department had offered to help curb the risk from the incident. Lockheed is the maker of the F-16, F-22 and F-35 fighter jets as well as warships and other multibillion-dollar arms systems sold worldwide. Analysis There was no word on what information may have been compromised in the attack nor where it may have originated. Military contractors' systems contain technical specifications on weapons under development as well as those currently in use. The US government has offered to help Lockheed analyse "available data in order to provide recommendations to mitigate further risk," Chris Ortman, a DHS official, said in an emailed reply to a query from Reuters. A person with direct knowledge said that unknown attackers had broken into sensitive networks of Lockheed and several other US military contractors. Boeing Co and Northrop Grumman, the Pentagon's No 2 and No 3 suppliers respectively, declined to discuss matters involving corporate security. Several top cybersecurity experts with extensive government dealings said they were in the dark about the origin of the attack.