It is far from enough for Japan to simply express regret over an incident involving a Self-Defense Forces officer illegally breaking into the Chinese Embassy in Japan, the Foreign Ministry said on Friday.

Lin Jian, spokesman for the ministry, urged Tokyo to conduct a thorough investigation and provide Beijing with a responsible response.

Japan's Defense Minister Shinjiro Koizumi said on Friday that it was deeply regrettable that a Self-Defense Forces officer, who is expected to comply with the law and maintain discipline, had been arrested for breaking into the Chinese Embassy in Japan.

Lin revealed chilling details of the security breach, noting that the individual illegally entered the embassy grounds armed with a 31-centimeter-long knife during commuting hours, and hid in the bushes for an extended period of time.

"Who was he waiting for by setting such an ambush? What did he plan to do?" Lin asked, noting that Japan has yet to provide answers to these critical questions.

He also condemned attempts by Japanese right-wing activists to whitewash the illegal action.

He added that while some activists initially claimed the individual could not possibly be a member of the Self-Defense Forces, "the facts speak louder than their words".