China's coast guard vessels were seen sailing in the territorial waters around disputed islands in the East China Sea on Saturday, the second time in a week, Japanese coast guard said.
According to Japan, four Chinese ships entered the waters surrounding the island chain, controlled by Japan and known as Senkaku in Japanese and Diaoyu in China at around 10.30 am (9.30 am Singapore time). However, they left the territorial waters within two hours.
On Sunday, Tokyo lodged a complaint with China's foreign ministry after a similar incident happened. Japan said the uninhabited islets are "an inherent territory of Japan".
Japan has repeatedly complained through diplomatic channels about the increasing number of Chinese ships around the island chain that has escalated the regional tensions.
The conflict between Japan and China has been rising due to "China's incursions into the waters of the Tokyo-controlled disputed islands". Since August 5, Japan has lodged at least 32 protests regarding 30 intrusions by Chinese vessels in the territorial waters.
The disputed islands are believed to harbour vast natural resources below their seabed and both the nations claim them as its own.
China is also involved in maritime disputes in the South China Sea by claiming a vast portion of the waterway, which Philippines, Brunei, Malaysia, Taiwan and Vietnam also claim.
A UN-backed tribunal in July ruled that China's vast claims were invalid but still China vows to protect its sovereignty over the region.