The Washington Post has reported that Chinese hackers allegedly accessed Japan’s defence networks, spurring concerns.

Tuesday’s comments come a day after the Washington Post cited unnamed current and former US and Japanese officials in an article alleging Chinese military hackers gained access to Japan’s classified defence networks in 2020.

According to the Post, the hackers obtained information about Japan’s military capabilities, plans and assessments of shortcomings. Japan’s Chief Cabinet Secretary Hirokazu Matsuno told a regular press briefing on Tuesday that Japan could not confirm whether any security information had been leaked.

The Post quoted one former US military official as saying the breach was “bad – shockingly bad” and that the head of the US National Security Agency flew to Tokyo to brief the Japanese defence minister, who asked the officials to also alert the prime minister.

The paper said Japan had taken steps to strengthen its networks. But it said that unnamed officials deemed these measures not sufficient to protect from spying by China.

A Pentagon spokesperson, Sabrina Singh, declined to comment on the Post report, saying it would let Tokyo speak for its intelligence and cyber capabilities. However, she added: “We feel confident in our relationship and the intelligence sharing that we do with Japan and we’re confident that we will continue that.”

The US National Security Agency did not immediately respond to a request for comment on the Post report, which comes ahead of a trilateral summit between the US, Japan and South Korea on August 18. The three allies are expected to announce plans to step up security cooperation in the face of increasing concerns about China.

Asked about the Post report, Japan’s Chief Cabinet Secretary Matsuno said Japan and the US have always been in close communication on various levels.

He added: “Due to the nature of the matter, I am unable to provide further details of the communication but we haven’t confirmed the fact that security information has been leaked due to cyber attacks.”

Matsuno said cybersecurity was the foundation for maintaining the Japan-US alliance and that Japan would continue to work to keep its network firm and secure.

There was no immediate comment from Beijing.

The US, Japan and South Korea have been working on a joint statement for the summit to bind them more closely together on security issues, a Washington official said last week.

The official said the allies were discussing installing a trilateral leader-level hotline and are expected to unveil other measures, including strengthening trilateral exercises and moves to boost cooperation on cybersecurity, missile defence and economic security.