About 1.25 million personal records have been leaked after a cyberattack on the management system for Japan's pension service.
The number of cases might increase, Japan Pension Service President Toichiro Mizushima said at a news conference Monday. The records include people's names, pension numbers, addresses and birthdates.
The leak was discovered May 28 after a virus attached to e-mails was opened on employees' computers, Mr. Mizushima said.
The pension service is responsible for the operations of Japan's social insurance and pension systems.
The attack raises concerns about Japan's ability to protect personal data, and risks hurting Prime Minister Shinzo Abe politically. Public anger over the loss of 50 million pension records due to mismanagement during his first term as premier in 2007 contributed to his ouster.
Japan and the U.S. vowed Sunday to work closely together against cyberthreats, the allies said in a joint statement. They said cyberattackers increasingly are willing to demonstrate their intent and ability to harm information systems and infrastructure.