Beijing sent fighter jets to the East China Sea after Japanese aircraft followed a Chinese plane in a territorial dispute, the defence ministry said Friday according to state media.

A ministry official told a press conference that two J-10 fighters flew to the area on Thursday to monitor two Japanese F-15 fighters that had trailed a Chinese Y-8 aircraft, China's official Xinhua news agency said.

The comments came after Japanese media reported Tokyo had scrambled fighter jets to head off an unspecified number of Chinese military planes near islands at the centre of an increasingly tense maritime row.

The two countries are at odds over the small, uninhabited islands controlled by Tokyo as the Senkakus but claimed by Beijing, which calls them Diaoyu.

The Beijing defence ministry official said that Japanese military planes have been increasingly watching Chinese aircraft and have also extended the areas where they are active, Xinhua reported.

China's military will be on high alert and the country will protect its air defence force, the official said.

Japan's Fuji TV network quoted Tokyo officials as saying that Chinese planes were spotted Thursday on Japanese military radar north of the islands.

The aircraft did not violate territorial airspace over the islands but flew inside Japan's so-called air defence identification zone, the report said.

Tokyo's defence ministry has said that F-15s were sent airborne to head off Chinese state-owned -- but not military -- planes four times in December, including an occasion when Japanese airspace was breached.

They were also mobilised once last week, it said.