Chinese nation-state hackers had breached Japan's classified defense network in 2020 in what was described as one of the most harmful hacks in Japan's modern history, The Washington Post reported, citing former US and Japanese officials familiar with the matter.

The hacks that began under the Trump administration and continued into the Biden administration were detected by the US National Security Agency (NSA) and were reported to Japanese government officials.

According to the Post, the intruders had deep, persistent access and appeared to be after anything they could get their hands on — plans, capabilities, and assessments of military shortcomings.

US Cyber Command initially offered Japan a team of cyber experts to help assess the scope of the breach and purge the Chinese malware from the compromised networks but the offer was turned down because Japan reportedly was wary of another nation's military accessing their systems.

Japan plans to adopt a more active national security strategy, which is said to include a new cyber command to monitor networks 24/7, with plans to spend $7 billion over five years on cybersecurity and add as many as 4,000 active cybersecurity personnel, according to the Post.

For its part, Japan refused to comment on the Post’s report.

“I won’t comment on the details due to the nature of the matter,” Defense Minister Yasukazu Hamada told reporters. “We have not confirmed that classified information held by the Defense Ministry has been leaked due to a cyberattack.”

Earlier this week, reports emerged that two North Korean hacker groups compromised the internal systems of the major Russian missile engineering company.