The Chinese embassy in Japan has received a series of “terrorist threats”, including an intrusion and a bomb threat, in the past month, a diplomat said on Thursday, describing the incidents as “extremely egregious”.

Shi Yong, the acting head of the embassy, said that on March 5 the embassy received a threatening letter from an organisation claiming to comprise former Japanese police officers and former Self-Defence Forces members.

Shi said the embassy had reported the matter to the local police, but said the Japanese police did not take matters seriously, nor did they adopt effective measures. He said the facts behind the correspondence were still not known.

Nineteen days later, Kodai Murata, a second lieutenant with the Ground Self-Defence Force, broke into the Chinese embassy compound by climbing over a wall while carrying a knife, Shi said. China lodged a protest with Japan’s Ministry of Foreign Affairs and the police.

Shi also said that on March 31 a separate individual claiming to be a Japan Self-Defence Forces emergency reserve officer issued a terrorist threat online, stating that he had planted a remotely detonated bomb inside the Chinese embassy.

Shi said the embassy immediately alerted police, who conducted a two-hour bomb search. On Thursday, he did not say whether any device was found.

“These incidents constitute a grave violation of international law, a serious infringement upon China’s sovereignty and dignity, and a threat to the safety of Chinese diplomatic personnel and premises, and are extremely egregious in nature,” he said.