The computer servers of eight ministries and organizations of the Japanese government, including the Prime Minister’s Official Residence and the Defense Agency, were hit by cyber attacks on two occasions between Sunday and Tuesday.

Government sources said Wednesday that the attacks temporarily cut off access to their Web sites.

The origin of the attacks is unknown, but they are believed to have been denial-of-service, or DoS, attacks aimed at bringing a computer network to a halt by flooding its server with massive amounts of useless data.

Also attacked were the servers of the Foreign Ministry, the Finance Ministry, the National Police Agency, the Justice Ministry’s Immigration Bureau, the Fair Trade Commission and the Japan Coast Guard, the sources said.

Huge amounts of data were sent to the servers between 9 p.m. Sunday and 2 a.m. Monday, and between 10 a.m. and 4 p.m. Tuesday, overloading the networks and making the Web sites inaccessible, they said.

In July, an online bulletin board in China called for people to target Japanese central government agencies in cyber attacks, naming the Prime Minister’s Official Residence, the Foreign Ministry, the Finance Ministry and the NPA. It is unclear whether this message was related to this week’s attacks.

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