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) makes everything from warships to missiles. The giant company says it discovered that viruses were found on at least 80 of the thousands of its servers and computers in mid-August and informed the Japanese police at the end of the month, the BBC reports.
The government said it was not aware of any leak of sensitive information.
But the defence ministry has demanded that MHI carry out a full investigation. Officials were angered after learning of the breach from local media reports.
Speaking at a news conference on Tuesday, defence minister Yasuo Ichikawa said the cyber attackers had not succeeded in accessing any important information but MHI would be instructed "to undertake a review of their information control systems".
"The ministry will continue to monitor the problem and conduct investigations if necessary."
All government contractors are obliged to inform ministers promptly of any breach of sensitive or classified information.
An ongoing internal investigation has found only network information, such as IP addresses, has been compromised.
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, MHI said.
They have been described as spear phishing attacks - when hackers send highly customised and specifically targeted messages aimed at tricking people into visiting a fake webpage and giving away login details.
Neither the Japanese government nor MHI have said who may be responsible.
