China's use of an armada of fishing boats — part of its so-called maritime militia — near the disputed Senkaku Islands could spell trouble for Tokyo as Beijing ramps up its assertiveness in the East China Sea, experts say.

On Sunday morning, Tokyo lodged a protest with the Chinese Embassy after two China Coast Guard ships entered what Japan calls its territorial waters around the uninhabited islands that it administers. Seven other Chinese ships were meanwhile spotted in the so-called contiguous zone just outside, the Japan Coast Guard said.

A day earlier, Tokyo protested to Beijing after seven Chinese Coast Guard ships and about 230 fishing vessels, an unusually high number, sailed close to the territorial waters around the islets, which are also claimed by China and Taiwan.