Japan's defense chief said Tuesday that the country's largest arms contractor has suffered a cyberattack, but that no sensitive information is known to have been lost.
Defense Minister Yasuo Ichikawa urged Mitsubishi Heavy Industry Ltd. to strengthen its data security systems.
"We are not aware of any important data having leaked to the outside," Ichikawa said.
The company makes ships, submarines, missile parts and other weapons for Japan's military.
Mitsubishi Heavy said it had been attacked in August with viruses apparently programmed to breach its computers and servers to gain unauthorized access to protected data. It did not say why it waited until this week to disclose the attack.
"We discovered that there had been a possible leak of system information such as network addresses from some of the computers at the company," it said in a statement Monday.
It said it has been working with police and independent experts to contain the damage.
The attack involved more than 80 servers and computers at 11 of the company's facilities related to nuclear power, missiles and submarines, the Nikkei business newspaper said Tuesday.
Attackers allegedly used simplified Chinese characters _ the writing system used in mainland China _ to remotely control the infected computers, and authorities are investigating the case as suspected spying, the nationwide Yomiuri newspaper reported Tuesday, quoting unidentified sources.
In Beijing, Chinese Foreign Ministry spokesman Hong Lei denied China was involved. "Criticism that China initiated a cyberattack is not only groundless, it goes against development of international cooperation on cybersecurity," Hong told a daily briefing.
An annual U.S. assessment of China's military in August said a number of computer systems, including U.S. networks, were the target of intrusions in 2010 that appeared to originate in China. The breaches were aimed at stealing data, but also exhibited the kinds of skills needed for more destructive network attacks, it said.
Another Japanese defense contractor, shipmaker IHI Corp., also reportedly came under cyberattack in recent months, receiving a number of emails with virus-loaded attachments.
Separately, Japan's National Police Agency said Tuesday that online messages were circulated last week in China calling for attacks on Japanese government websites ahead of the 80th anniversary of the Sept. 18 "Mukden Incident."
The 1931 event led to the Japanese occupation of China's northeast and eventually the invasion of much of the country. The date has in the past been marked by official commemorations and scattered anti-Japanese protests.
The police agency said several government websites were temporarily disrupted over the weekend, without linking the outages to the messages.
Chief Cabinet Secretary Osamu Fujimura said the series of computer problems underscored the need for the government to "further strengthen its information security measures."