Patrols in sovereign waters 'are regular missions': Foreign Ministry

Cheng Yonghua, China's ambassador to Japan, rejected a protest by Tokyo over Chinese vessels patrolling waters off the Diaoyu Islands.

Japanese Deputy Foreign Minister Saiki Akitaka summoned Cheng in Tokyo on Tuesday to lodge a protest against patrols conducted by four Chinese maritime surveillance ships.

The four vessels entered waters off the islands before noon on Monday and remained there for more than 13 hours, the Japanese coast guard said.

Relations between the two countries have sunk to their lowest level in years since the Japanese government illegally "purchased" part of the Diaoyu Islands in September. The islands belong to China and have been Chinese sovereign territory for centuries.

Li Xiushi, a researcher on Japanese studies at the Shanghai Institutes for International Studies, said the lack of trust between Tokyo and Beijing may further damage economic ties.

Beijing also blasted Japan on Tuesday for allowing fighter jets to violate the islands' airspace and letting its vessels enter China's territorial waters off the islands.

Foreign Ministry spokesman Hong Lei said China's patrols are regular missions conducted for administrative purposes and Beijing has lodged protests to Tokyo on a number of occasions demanding a halt to illegal activities.

Japanese Defense Minister Itsunori Onodera told his US counterpart Leon Panetta on Tuesday that countermeasures will "have to be taken'' in case of "provocations'' from China.Tokyo is eager to boost collaboration with Washington and enhance the "deterrence'' of Japan's military capability, Onodera said.

Japan may face consequences, Li warned. "If Japanese Prime Minister Shinzo Abe fails to be pragmatic, Japan may face challenges both domestically and overseas."