Japan on Thursday said it scrambled fighter jets after a Chinese vessel sailing in disputed waters launched a drone, the latest bout of tension in a longstanding territorial row.

The two countries are at odds over a group of small uninhabited islands in the East China Sea administered as the Senkakus by Japan but claimed as the Diaoyus by China.

Yoshihide Suga, Japan's top government spokesman, told a regular briefing that the incident took place Thursday and that Japan lodged a "strong protest", accusing China of "unilaterally escalating" tensions.

Japan dispatched four planes, including two F-15 fighters and an airborne warning and control system (AWACS) plane to waters surrounding the islets, Japan's defence ministry said.

Coast guard ships from the two nations routinely patrol the area, known for rich fishing grounds and possible energy reserves, to back up their claims.

The countries have also occasionally engaged in potentially dangerous encounters in the air and at sea over the dispute but have avoided actual conflict.

The latest incident came as the Japan Coast Guard said four Chinese ships entered Japanese territorial waters Thursday.

"This is the first time we see what appears to be a drone flown by Chinese ships that are sailing within territorial waters," Suga said.

"This is a new form of activity by China," he added.

"We strictly protested that this is a unilateral escalation and that we absolutely cannot accept it." 

Japan has said its military scrambled a record number of jets last year, mostly in response to an increase in approaching Chinese aircraft.

Tokyo ordered its jets to the skies 1,168 times in the fiscal year through March, with 73 percent of the missions against aircraft either known to be or believed to originate from China.

Japanese officials believe China has been trying to analyse the capacity and response patterns of Japan's defence and coastguard personnel.