Japan's defence minister on Tuesday said cyber attackers who breached security at defence contractor Mitsubishi Heavy Industries had not got their hands on any sensitive data.
"We are not aware of any important data being leaked," Yasuo Ichikawa Ichikawa told a news conference. "The ministry has business ties with the company, so will instruct them to undertake a review of their information control systems."
"The ministry will continue to monitor the problem and conduct investigations if necessary," Ichikawa added.
The company, which makes warships, submarines and other defence-related equipment, said Sunday that 45 Japan-based servers and 38 computer terminals were infected with viruses during a cyber attack in mid-August.
The machine that attackers used to remotely operate the affected computers appeared to be displaying Chinese language script, the Yomiuri daily reported in its Tuesday evening edition, citing unnamed sources.
Another defence contractor and heavy industry firm IHI said it had also come under attack, receiving an increasing number of e-mails carrying virus-laden attachments over the last few months.
"We have been able to prevent any real damage and infection. We are checking the health of our IT system just as precaution," an IHI spokesman said.
IT security firm Trend Micro said it had uncovered a campaign of targeted attacks that have successfully compromised defence industry companies in Japan, Israel, India and the United States.
"We have been able to identify eight victims of this attack and are in the process of notifying them," the company said in a blog dated Monday.
"In total, the attackers compromised 32 computers; however, there were multiple compromises at several locations. This network has been active since July 2011 and is continuing to send out malicious documents in an attempt to compromise additional targets."
On Monday Kyodo news agency, citing national police, said the websites of a number of Japanese government agencies had been attacked over the weekend, temporarily blocking access to them.
The attacks affected the websites of the National Personnel Authority, a video distribution service and a site run by the Cabinet Office, the National Police Agency said, according to Kyodo.
Police said they believed the websites were hit by distributed denial-of-service attacks in which hackers overwhelm a target website with massive amounts of data using a multitude of compromised "zombie" computers.
The NPA said online messages were posted in China calling for attacks on Japanese websites ahead of Sunday, the 80th anniversary of the "Mukden incident" which led to the Japanese invasion of China.
China has been accused of spearheading online attacks on government agencies and companies, allegations Beijing has always denied.
In June, Internet giant Google said a cyber-spying campaign originating in China had targeted Gmail accounts of senior US officials, military personnel, journalists and Chinese political activists.
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