The government has lodged a protest with Beijing after it found a Chinese drilling ship moored and engaging in suspicious activity near gas fields in the East China Sea, Foreign Minister Fumio Kishida said Tuesday.

"It is extremely regrettable that China is maintaining its activities toward the unilateral development of the area despite our repeated protests" and at a time when the maritime boundary between the countries in the East China Sea has yet to be fixed, Kishida told reporters.

There is a "high possibility" that the activities by the Chinese vessel are "trial digging toward construction of a new drilling facility," a Foreign Ministry source said.

Japan has repeatedly called on China to halt its resource development program near the Japanese-proposed median line separating the two nations' exclusive economic zones.

Sixteen above-water exploration platforms have been set up by Beijing in the East China Sea near the median line. China does not recognize the line and claims its EEZ stretches further to the edge of the continental shelf, closer to Okinawa Prefecture.

The Japan Coast Guard issued an alert last Friday for vessels sailing in the waters, warning them to take caution as a "drilling device" had been spotted in the East China Sea, in an apparent reference to the Chinese drilling ship.

The incident comes after the Japanese government said Oct. 12 that flares were witnessed at two new locations among the 16 exploration platforms, in an indication China is continuing to develop gas fields.