Tokyo has summoned Beijing's ambassador after Chinese military aircraft locked radar onto Japanese jets, the latest incident in the row ignited after Prime Minister Sanae Takaichi's comments backing Taiwan.
Takaichi suggested last month that Japan would intervene militarily in any Chinese attack on the self-ruled island that Beijing claims as its own and which it has not ruled out seizing by force.
No damage or injury was caused, but it was the first time that Japan had disclosed such an incident.
Fighter jets use their radar for fire control to identify targets as well as for search and rescue operations.
Japan had scrambled its F-15 jets because it was worried about possible "airspace violations", chief government spokesman Minoru Kihara said Monday.
China's navy said Sunday that the Japanese planes "repeatedly approached the Chinese Navy's training area and caused trouble, seriously affecting the normal training of the Chinese side and seriously endangering flight safety."
A statement said that Tokyo's claim was "completely inconsistent with the facts" and told Japan to "immediately stop slandering and smearing".
Kihara responded on Monday that China's "claim that the Self-Defence Forces aircraft seriously obstructed the safe flight of Chinese aircraft is unfounded."
Vice Foreign Minister Takehiro Funakoshi summoned Ambassador Wu Jianghao on Sunday and "made a strong protest that such dangerous acts are extremely regrettable".
Funakoshi "strongly urged the Government of China to ensure that similar actions do not recur," the Japanese foreign ministry said late Sunday.
Takaichi said the same day that Japan would "respond calmly and resolutely."
Beijing's foreign ministry stated that it rejected the protest and had lodged its own counter-protest, according to the state news agency Xinhua.
