A recent cyber attack against Mitsubishi - one of Japan's biggest defense contractors - likely compromised military data on warplanes and nuclear power plants.
However, as Japan's Asahi newspaper reports, it remains unclear whether the netted information was sensitive in nature.
Although Mitsubishi declined to comment on the matter, a Japanese defense ministry official told Reuters the government bureau is "not aware" of any information leaks that can be classified as "defense secrets."
Meanwhile, defense ministry sources told the Dow Jones Newswire that data on Mitsubishi's fighter aircraft was transferred from one internal server to another due to "computer viruses," but emphasized it has yet to be confirmed if the data was transmitted outside the company.
Similarly, sources in the Ministry of Economy, Trade and Industry said data pertaining to nuclear reactors may have been transferred between company servers, yet noted there was "little chance" confidential nuclear reactor data was leaked.
In August, Mitsubishi confirmed its computer network had been attacked, with 45 servers and 38 computers falling victim to over 50 types of viruses at 11 locations in Japan - including the company's plant in Aichi Prefecture which manufactures missiles and aircraft engines.
A number of the affected servers and computers were forcibly connected to websites abroad, resulting in the loss of "some data," including (unspecified) IP addresses.