Speaking to domestic media, an official from the defence ministry said Japanese military aircraft have been increasingly active in closely scouting Chinese aircraft.
“The activity zone of Japanese military aircraft has also expanded recently, which is the root cause of security disputes concerning territorial waters and airspace between the two countries,” the state-run Xinhua news agency quoted the ministry official as saying.
The Chinese military will be on high alert and China will resolutely protect the security of its air defense force and uphold its legitimate rights, the official said.
The official also called for the Japanese side to respect relevant international laws and to prevent security disputes by taking effective measures.
Foreign Ministry spokesperson, Hong Lei, said Chinese military planes were on "routine flights" in relevant airspace over the East China Sea.
Hong Lei made the remarks at a press briefing in response to media reports that Japan sent fighter jets to head off a number of Chinese military planes spotted in Japan's "air defense identification zone" over the East China Sea on Thursday.
"China firmly opposes Japan's moves to gratuitously escalate the situation and create tensions," Hong said.
“Relations between Japan and China have soured since the Japanese government said it would "purchase" part of the Diaoyu Islands in the East China Sea in 2012. China insists that the islands are part of its inherent territory,” Xinhua reported.