Tokyo, Japan - Chinese military aircraft locked radar onto Japanese fighter jets southeast of Okinawa's main island, Japan's defense ministry said on Sunday.

China's navy said Tokyo's claim was "completely inconsistent with the facts" and told Japan to "immediately stop slandering and smearing".

Relations between Tokyo and Beijing have soured following remarks by new Japanese Prime Minister Sanae Takaichi suggesting that Tokyo could intervene militarily in any attack on Taiwan.

Chinese J-15 fighter jets twice locked radar on Japanese fighter jets on Saturday, without causing damage or injuries, Japan's defense ministry said.

Fighter jets use their radar for fire control to identify targets as well as for search and rescue operations.

Defense Minister Shinjiro Koizumi said the incident was "dangerous and extremely regrettable", adding that Japan had lodged "a strong protest" with China.

The two countries have a long-running territorial dispute over the Senkaku Islands, known as the Diaoyu in China.

The tiny, uninhabited islands lie between Okinawa and Taiwan, the much larger self-ruled island that China also claims.

Beijing summoned Tokyo's ambassador following Takaichi's comments last month.

Tokyo is deepening cooperation with US allies in the Asia-Pacific region, where several countries have territorial disputes with China.

Beijing, for instance, claims nearly all of the South China Sea, and has been asserting control more strongly in parts of the strategic waterway despite an international ruling that its claim has no legal basis.