Japanese defence contractor Mitsubishi Heavy has been hit by over 50 separate computer viruses in a series of cyber attacks so far this year, a report said Monday.

One of the infected computers contracted 28 types of virus, including ones that could have been prevented with existing anti-virus software, the Yomiuri Shimbun said, amid concerns over the security of defence information.

The defence ministry plans to tell Mitsubishi Heavy to file detailed reports on its cyber security measures, the newspaper said.

Mitsubishi Heavy is analysing 83 computers hit by the attacks and has found more than 50 kinds of viruses, significantly more than the eight strains that the company announced on September 19, the newspaper said.

The company declined to comment for the Yomiuri story, citing an ongoing police investigation, while Mitsubishi officials could not be reached on Monday -- a national holiday -- for immediate comment.

Japan's defence minister said in September there was no indication that sensitive information had been stolen as a result of the attacks on the firm's computers, which came to light in August.

The Yomiuri reported earlier that the attackers might have used Chinese-language script in creating the viruses.

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.

After US defence contractors were targeted by cyber attacks earlier this year, Deputy Defense Secretary William Lynn claimed that a foreign intelligence agency was behind an attack that stole classified information on a top secret weapons system.