Japan has protested after Chinese fighter jets locked radars on Japanese aircraft as tensions between the two nations worsened.

Locking radar onto an aircraft is considered a threat because it can signal a potential attack. Japan said there were two such incidents Saturday off its southern Okinawa islands.

Japan said it scrambled fighter jets in response to the Chinese J-15 fighter jets, while Beijing accused Tokyo of "harassing" its forces during a training exercise. No injuries or damage were reported.

Diplomatic ties between Japan and China have spiralled since last month, after Japanese Prime Minister Sanae Takaichi suggested that Tokyo could take military action if Beijing attacked Taiwan.

Beijing views self-governed Taiwan as part of its territory and has not ruled out the use of force to "reunite" with it.

Both sides have since engaged in increasingly hostile rhetoric towards each other, with the widening rift affecting daily life for citizens in both countries.

Last week, China and Japan's coast guards gave conflicting accounts of a confrontation near disputed islands in the East China Sea.

A Japanese defence ministry official said the intention of the Chinese J-15 jets was "unclear", but added that there was "no need" to lock on to the Japanese planes if their intention was to locate other aircraft.

The J-15 jets, which were launched from China's Liaoning aircraft carrier, first locked its radar on Japanese jets at 16:32 local time on Saturday (07:32 GMT) and again at around 18:37.

The official added that the Japanese aircraft "did not do anything that could be considered a provocation".

"It is extremely regrettable. Japan has strongly protested to the Chinese side, and we firmly requested measures to prevent recurrence," Takaichi told reporters Sunday in Wajima, Ishikawa prefecture.

"We will respond calmly and resolutely," she said.

The Chinese navy however said Japan's claim was "completely inconsistent with the facts" and told Tokyo to "immediately stop slandering and smearing". It added that its training exercise in the area had been previously announced.

This comes two weeks after Japan scrambled aircraft when a suspected Chinese drone was detected off Yonaguni, island near Taiwan. Tokyo has said it is planning to deploy missiles from Yonaguni in a move that has angered Beijing.

A month of heightened tensions have seen China ask its citizens to avoid travelling to Japan.

China has also banned the importation of seafood from Japan and suspended the screening of popular Japanese films.