Hackers have accessed information-sharing software developed by Japan's Fujitsu, resulting in data leaks from Japanese government offices.
Fujitsu's ProjectWEB software is widely used by public offices and businesses.
The hackers accessed the software at Narita Airport east of Tokyo and stole data on air traffic control, prompting the Cabinet Secretariat's national cybersecurity center to alert establishments that use the software.
On Wednesday, the land, infrastructure and transport ministry reported that at least 76,000 email addresses of its employees and business partners were leaked, along with data on the ministry's internal mail and Internet settings.
The cybersecurity center reported that data on the center's information system were stolen from several of its offices.
Officials at the center say they have taken steps to prevent attacks using leaked data, and that they will continue to step up their alert.
Fujitsu has suspended the software's services to investigate the scope of the attacks.
The company says it is consulting the affected offices and that it will fully support clients who have suffered damage.
Chief Cabinet Secretary Kato Katsunobu told reporters on Wednesday that no damage to the Cabinet Secretariat's cybersecurity center's system has been confirmed, and that the center's operations have not been compromised.
Kato added that cybersecurity must be tightened, as hackers could target the Tokyo Olympics and Paralympics.