Japan protested against China on Sunday after several Chinese coast guard vessels were seen sailing into territorial waters around the disputed islands in the East China Sea, Tokyo said.
According to Japan Coast Guard, four Chinese ships entered the waters surrounding the islets, called the Senkaku in Japan and the Diaoyu in China, at around 10 am local time (9 am Singapore). However, they left the territorial waters within two hours.
Japan has repeatedly complained about the increasing number of Chinese ships to the island chain that has escalated the regional tensions.
Tokyo lodged a protest to China's foreign ministry on Sunday through its embassy in Beijing. A government official told AFP that the lodged complaint pointed that the islands are "an inherent territory of Japan".
Prime Minister Shinzo Abe also ordered officials to beef up its team who were in charge of monitoring Chinese ships.
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.
But, China regards them as its own, rejecting the view it violates Japan's territorial waters.
China is also involved in maritime disputes in the South China Sea by claiming a vast portion of the waterway. The Philippines, Brunei, Malaysia, Taiwan and Vietnam also claim the same waterway.
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.