Beijing-Tokyo relations have soured over the past month, following remarks by Japanese Prime Minister Sanae Takaichi suggesting that Tokyo could intervene militarily in any attack on Taiwan.

Japanese Defence Minister Shinjiro Koizumi called the incidents "dangerous and extremely regrettable" at a hastily called news conference early Sunday, saying Japan had lodged "a strong protest" and told China to prevent a recurrence.

No damage or injuries were reported from Saturday's incidents, the ministry said.

In the first incident over the seas southeast of Okinawa Island, "a J-15 fighter jet launched from the Chinese Navy's aircraft carrier 'Liaoning' intermittently locked the radar" on a Japanese Air Self-Defense Force F-15 fighter jet that was scrambled over the Chinese aircraft's airspace intrusion, the ministry said in a statement.

About two hours later, a Chinese J-15 fighter jet from the Liaoning intermittently locked radar on another Japanese fighter jet, it said.

"The radar lock in these incidents constitutes a dangerous act that exceeds the scope necessary for the safe flight of aircraft," it said.

"It is extremely regrettable that such incidents have occurred," it said.

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