The break-in has further soured relations between Beijing and Tokyo that have suffered since comments by Japanese Prime Minister Sanae Takaichi about the self-ruled island of Taiwan.

China lodged a protest following the incident on Tuesday, in which Beijing's foreign ministry said the man threatened to kill diplomats.

Japan's top government spokesman Minoru Kihara said: "It is truly regrettable that a Self-Defense Forces member, who is expected to comply with the law, has been arrested on suspicion" of entering the embassy premises.

"Police are already conducting an investigation to clarify what happened and have implemented necessary measures to strengthen security" of the embassy, he told a press conference.

"We will take necessary steps to prevent any recurrence of such incidents," he added.

Separately, a police spokeswoman told AFP that the suspect, Kodai Murata, 23, was arrested Tuesday on suspicion of trespassing after he broke into embassy grounds around 9:00 am (0000 GMT).

The suspect told police investigators that he was "hoping to meet the ambassador to tell him to refrain from making hardline remarks, and if that request was rejected, I wanted to surprise him by killing myself," the Yomiuri Shimbun reported, citing unnamed sources.

Japan's national broadcaster NHK said a knife was later found at the scene.

Chinese foreign ministry spokesman Lin Jian said Tuesday that Beijing was "deeply shocked" by the incident and had "lodged solemn representations and a strong protest with the Japanese side".

Ties between Japan and China have deteriorated since Takaichi suggested in November that Japan might intervene militarily in any Chinese attempt to take Taiwan.

China, which regards the democratic island as part of its territory and has not ruled out force to annex it, was furious over the comments.