Japan's biggest defense contractor Mitsubishi Heavy has admitted that it was targeted by a major cyberattack last month in which at least 83 of its computers were infected with malware.

According to reports in the Japanese press, 45 servers and 38 PCs across 10 facilities in Japan were successfully hit by a range of malware, including Trojans designed to record and steal password logins.

Sites attacked ranged across the company's wide-ranging portfolio of defense and industrial businesses, and included shipyards and subsidiaries involved in the production of guidance and rocket propulsion systems for missiles.

The company said it believed no significant data had been compromised beyond the IP addresses of certain network resources, which was first noticed had been probed along with the servers on August 11.

"We can't rule out small possibilities of further information leakage but so far crucial data about our products or technologies have been kept safe," a Mitsubishi heavy spokesperson was quoted as saying by Reuters.

The scale and targeted nature of the attack will prompt comparisons with similar attacks against US industrial and military servers during the last two years that have been blamed almost entirely on Chinese cybercriminals.

Attacks discovered this year alone have included the 'Night Dragon' attacks on a clutch of mainly US energy companies, one on the US Oak Ridge National Laboratory, and a sizable campaign against a number of US defence contractors that became public knowledge in June. As with a separate attack on Google during the same time period, all were blamed on China.