Lockheed Martin last weekend acknowledged that its information systems network had been the target of a "significant and tenacious attack".
The aerospace and defence giant said that its security team detected the intrusion "almost immediately and took aggressive actions to protect all systems and data".
No data from customers, programmes or employees was compromised, the top company said in a brief statement.
The company said that it has kept US government agencies informed of its investigation as it "continues to work around the clock to restore employee access to the network."
The attack was first reported last Thursday by Reuters, which cited a defense official and two unnamed sources familiar with the situation. The news agency reported that the cyberattack was affecting many employees at Lockheed Martin, which is based in Bethesda, Maryland, and makes fighter planes and other weapons systems.
On Saturday, an official with the U.S. Department of Homeland Security confirmed the attack to the news agency. However, Lockheed Martin continued to decline comment.
The intrusion reported involves the use of RSA SecurID tokens, used by Lockheed Martin employees to access the company network remotely. Security analysts have urged that companies using the tokens review authentication procedures.
Lockheed Martin did not divulge how its systems were attacked. The company faces "constant threats from adversaries around the world" and regularly acts to heighten security of its systems, it said in the statement.