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.

A Chinese carrier strike group launched intense air operations near Japan over the weekend as the East Asian neighbours traded diplomatic barbs in an escalating dispute, further straining ties.

Japan on Sunday summoned China's ambassador, Wu Jianghao, to protest "dangerous" and "regrettable" acts by the carrier's planes.