Computers at several of Japan's overseas diplomatic missions have been hit by cyber attacks, a report said Wednesday, just a day after it was revealed the country's parliament had been targeted.

Computers at embassies and consulates in nine countries were infected with viruses in the summer, the Yomiuri Shimbun said, citing unnamed sources.

An official for the foreign ministry said no classified information had been stolen and there appeared to be nothing out of the ordinary about the viruses.

"Virus infections are not a recent phenomenon, but have been experienced for a long time," he said.

"We have got rid of the viruses every time and no problem has occurred," he told AFP by phone.

He said the infections had been found in the ministry's Internet-capable open systems, while diplomatic secrets are handled in a closed system.

The Yomiuri said the countries where infection was found included France, the Netherlands, Myanmar, the United States, Canada, China and South Korea.

The paper said the infection included a "backdoor" virus, which opens a route into a computer's system to allow access by a remote hacker, who could use it to steal data.

On Tuesday the Asahi Shimbun 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 the Asahi on Monday said 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.