A virus that infected computers at Japanese overseas diplomatic missions had been designed to send data to servers in China, a report said Friday.
The virus -- Backdoor Agent MOF -- has been found to have infected computers at around 10 embassies and consulates, and at least two of the servers designated as the recipients of stolen information were in China, the Yomiuri Shimbun said.
The virus is capable of transmitting user IDs and other information to terminals outside and operating software by bypassing authorised users, the daily said.
The domain of the servers was the same as that used for earlier cyber attacks on Google and tens of other companies, the Yomiuri said, quoting unnamed sources.
A "backdoor" virus opens a route into a computer's system to allow access by a remote hacker, who could use it to steal data.
The Yomiuri earlier this week reported Japan had found viruses in computers at overseas diplomatic missions including those in France, the Netherlands, Myanmar, the United States, Canada, China and South Korea.
The government has admitted virus infections at some offices but said no classified information had been stolen.
On Friday, government spokesman Osamu Fujimura again said no sensitive information had been compromised, but refused to comment on the specifics of the Yomiuri report.
"I'd like to withhold comment on what kind of viruses they were or where they came from," Fujimura told reporters.
The Asahi Shimbun this week reported that computers in the lower house of parliament were hit by cyber attacks from a server based in China that left information exposed for at least a month.
That revelation came after a probe began into attacks on defence contractor Mitsubishi Heavy, which could have resulted in the theft of information on military aircraft and nuclear power plants.
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 the Gmail accounts of senior US officials, military personnel, journalists and Chinese political activists.
It was the second time that Google has reported a China-based cyber attack. Early last year, a similar incident prompted the company to reduce its presence in China.
Explore further: Digital dilemma: How will US respond to Sony hack?