|The defence ministry has demanded Mitsubishi Heavy Industries carry out a full investigation [EPA]
Japan's top weapons maker has confirmed it was the victim of a cyber attack reportedly targeting data on missiles, submarines and nuclear power plants.
Mitsubishi Heavy Industries (MHI) confirmed on Tuesday that 45 of its servers and 38 computers were infected by at least eight viruses.
The government said they were not aware of any sensitive leaking of information from the attacks.
But the defence ministry has demanded MHI carry out a full investigation after learning of the breach from local media reports.
All government contractors are obliged to inform ministers promptly of any breach of sensitive or classified information.
"It's up to the defence ministry to decide whether or not the information is important. That is not for Mitsubishi Heavy to decide. A report should have been made," a defence ministry spokesman was earlier quoted by Reuters news agency as saying.
MHI, which has built the US designed F-15 fighter jet and missile systems including Patriot batteries
under licence, said that computer systems had been accessed in August and some network information, such as IP addresses, may have been leaked.
Should MHI probe reveal the loss of sensitive data, the defence ministry could impose penalties on its main domestic arms supplier as under the terms of an agreement the government imposes on all contractors.
MHI had won 215 deals worth $3.4bn from the Japanese defence ministry in the year to last March.
Yasuo Ichikawa, the Japanese defence minister, said he had so far received no reports of classified information having been looted in the online assault. He did not say what information was at risk.
The online attacks - which are believed to be the first of their kind against Japan's defence industry - originated outside the company's computer network.
Emails containing viruses
A MHI spokesman declined to comment further on the first known cyber attack on Japan's defence industry, saying it aims to conclude an investigation by the end of September.
A second defence contractor, IHI, which supplies engine parts for military aircraft, said it had also been targeted.
IHI said it had been receiving emails containing viruses for months, but its security systems had prevented infection.
There are also reports that Japanese government websites, including the cabinet office and a video distribution service, have been hit by distributed denial-of-service attacks.