Japan's top government spokesman has expressed grave concern over Chinese naval moves near the Senkaku Islands, and vowed to respond calmly but firmly.
Chief Cabinet Secretary Yoshihide Suga was speaking on Thursday on the entry of a Chinese frigate and a submarine presumably from China into the contiguous zone just outside territorial waters around the islands.
Japan controls the islands. The Japanese government maintains that the islands are an inherent part of Japan's territory. China and Taiwan claim them.
Suga said he will refrain from commenting on the nationality and other details of the submerged submarine, given the nature of the matter.
He said Chinese government vessels have repeatedly entered Japanese territorial waters, and that the country has just sent a warship to the contiguous zone near the Japanese islands.
He expressed grave concern that the latest incident, which follows a similar case in June 2016, would unilaterally add to tension.
Suga said Japan is firmly and calmly responding to the issue to decisively defend Japan's territory, territorial waters and skies.
Suga also said Japan will seek to improve relations with China as agreed to in a series of summits last year. He said Japan will ask China not to hamper moves to better bilateral relations.
China meanwhile stressed that the country is resolved to maintain sovereignty over its territory.
Foreign Ministry spokesperson Lu Kang told a news conference on Thursday that the Chinese Navy was carrying out surveillance of Japanese Maritime Self-Defense Force vessels.
Lu said those islands are an inherent part of China's territory, and that there are plenty of historical and legal grounds for Chinese sovereignty over them.
He said Japan cannot change the objective fact that the islands belong to China. He said Japan must stop causing trouble over the issue.
Lu said Japan should try to narrow differences with China based on common understanding shared by both sides, and take action to improve bilateral relations.