Japan's parliament has come under cyber attack again, apparently from the same emails linked to a China-based server that have already hit several lawmakers' computers, an official said.

Malicious emails were found on computers used in the upper chamber of the Japanese parliament, a government spokesman said.

"The upper house office has confirmed that seven suspicious emails, the same ones that were sent to the lower house, were found" in computers in the upper house, Deputy Chief Cabinet Secretary Isao Saito said.

A report last week said that computers in the lower chamber had been hit by a virus, with passwords and other information possibly compromised.

But Saito said the email server of the upper house had not succumbed to any virus and security had been tightened on all machines used by lawmakers there.

Local media reported last month that politicians' computers and a lower house server had contracted a "Trojan horse" virus containing a program that allowed a China-based server to steal passwords and other information.

It was not clear who was behind the attack, the reports said, adding it was possible the China-based server could have been controlled from a third country.

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.

China angrily denies that it is orchestrating any online attacks on foreign government agencies and companies.

Japan is already probing a series of recent attacks on computer systems at defence contractor Mitsubishi Heavy, which reportedly could have resulted in the theft of information on military aircraft and nuclear power plants.

Computers at several of Japan's overseas diplomatic missions have also been targeted by hackers, Japanese media said last week.