Japan's National Police Agency chief said on Thursday he will resign to take responsibility for the murder of former Prime Minister Shinzo Abe, citing the need for a "fresh start" for the organisation and its security duties.

Itaru Nakamura is the most senior official to step down in connection with Abe's assassination at a campaign rally in the western city of Nara on July 8, where experts have said security was seriously flawed.

"In the process of verifying our new security plan, we have come to realise that our security duties would need a fresh start," Nakamura told a news conference.

"To mark our fresh start with a new security plan, it is only natural for us to build a new organization."

Security in Nara on the day of the shooting had been widely seen as insufficient, experts have said.

Bodyguards could have saved Abe by shielding him or pulled him from the line of fire in the 2.5 seconds between a missed first shot and the second, fatal round of gunfire, eight security experts who reviewed the footage have told Reuters.

The chief of the Nara police also announced his intention to resign, Japanese media said.

Japanese officials, including Prime Minister Fumio Kishida, have acknowledged flaws in the security around Abe's appearance at the election campaign event.

The National Police Agency previously told Reuters the killing had been the result of police failing to fulfil their responsibility, adding that it had set up a team to review security and protection measures and develop preventive steps.

The suspected assassin, arrested at the scene moments after the shooting, is undergoing psychiatric evaluation, Japanese media reported last month.