Mitsubishi Heavy's admission came on the same day that the Japanese newspaper cited unnamed sources who said data on company-built fighter jets, helicopters and nuclear power plants had apparently been stolen during the attack.

Last month, Mitsubishi Heavy said that among the 83 compromised servers and PCs were ones at its Kobe shipyards, where the company builds diesel-electric submarines and components for nuclear power plants; at the company's Nagasaki shipyards; and at its Nagoya plant, which designs and manufactures missile guidance systems.

The firm's corporate headquarters was also targeted by attackers.

Mitsubishi Heavy said it was continuing to investigate and would cooperate with Japanese authorities, who have been involved since late September when the company filed a complaint with the Tokyo Metropolitan Police Department, said Asahi Shimbun.

The August attack was the first against a major Japanese defense contractor, but followed others earlier this year aimed at U.S. companies, including , which manufactures the F-22 Raptor and F-35 Lightning II fighter aircraft. The Lockheed attack was carried out using information stolen earlier from RSA Security, the branch of EMC that produces the SecurID two-factor authentication token used by thousands of contractors and corporations to secure their networks.