A?Chinese foreign ministry?spokesman?Friday?urged Japan to protect the safety of Chinese embassy and personnel within its territory in response to media reports of threatening mail sent to the Chinese embassy in Tokyo.

The Chinese Embassy in Tokyo received an envelope containing a rifle bullet on Thursday and handed it over to the Japanese police, Yang Yu, spokesman for the embassy, confirmed on Friday afternoon.

The mail is now under further investigation, Yang said, adding that the Chinese embassy has demanded the Japanese police adopt effective measures to ensure the safety of Chinese organizations, companies and personnel in Japan.

Foreign ministry spokesman Hong Lei said that China had expressed grave concern over the incident, adding that China has asked Japan to take effective measures to guarantee safety for Chinese embassies and personnel in Japan in accordance with the Vienna Convention on Diplomatic Relations.

As Saturday marks the 40th anniversary of normalization of China-Japan relations, it should serve as an opportunity for the two countries to push forward their bilateral ties, Hong said.

However, the spokesman reiterated, the Japanese government's insistence on "buying" the Diaoyu Islands severely infringed upon China's territory and sovereignty, leaving a series of diplomatic activities postponed or canceled.

China-Japan relations face a grim situation at present, caused by Japan's unilateral action on the Diaoyu Islands, he said.

Such an action posing a grave challenge to post-war international order is against the will of both the Chinese people and people all across the world, according to Hong.

China hopes the Japanese side can realize and correct its wrongs and stop all the actions infringing China's sovereignty in order to help bilateral relations between the two countries come back to a healthy and stable track, Hong said.

(Xinhua contributed to the story)