The Defense Ministry and Self-Defense Forces were targeted by a sophisticated cyberattack in September that may have compromised the Defense Information Infrastructure, Japan's internal military network, ministry sources say.

The attacker, possibly a state actor, initially penetrated the Ground Self-Defense Force's computer system and likely stole some information, though the full scope of the theft remains unclear, the sources said Sunday.

The attacker took advantage of a security loophole, prompting the ministry to raise its cybersecurity alert level.