The attacks, late Sunday and early Tuesday, targeted eight ministries and agencies and caused computers to freeze up under a deluge of data, Chief Cabinet Secretary Hiroyuki Hosoda told a news conference.
Hosoda said the barrage also made it impossible for anybody to access Web sites for the eight government bodies--the Cabinet Office, Foreign Ministry, Finance Ministry, Justice Ministry, National Police Agency, Defense Agency, Coast Guard, and Fair Trade Commission
He said there was no significant damage and that the networks had resumed normal operations. However, he added that it's nearly impossible to track the data.
"We don't know where the attack came from, or who did it," he said.
In January, several ministries suffered a similar, small-scale cyberattack, temporarily freezing Web servers but causing no permanent damage.
So-called denial-of-service--or DoS--attacks bombard a Web server with so much data that the machine becomes unusable.