A Chinese Coast Guard ship pictured near disputed islets, known as the Senkaku islands in Japan and Diaoyu islands in China, in the East China Sea in December 2015 (File Photo: AFP)
Tokyo protested to Beijing Sunday after Chinese coast guard ships sailed into territorial waters surrounding disputed islands in the East China Sea, Japan said.
Four Chinese vessels entered the waters surrounding islets, called the Senkaku in Japan and the Diaoyu in China, around 10.00 am local time (0100 GMT), according to Japan Coast Guard.
They left the territorial waters within an hour, the coast guard said.
The two countries are locked in a long-running dispute over the uninhabited islets.
Tokyo has lodged at least 32 protests through diplomatic channels since August 5 over what it says have been about 30 intrusions by Chinese vessels in the territorial waters.
The director general of the Japanese foreign ministry's Asian and Oceanian affairs bureau, Kenji Kanasugi, issued a protest Sunday to the Chinese embassy in Tokyo, saying that the ships' "incursion" violated Japan's sovereignty, the ministry said in a statement.
"Despite Japan's repeated strong protests, the Chinese side has continued to take unilateral actions that raise tensions on the ground, and that is absolutely unacceptable," the statement added.
China is also involved in maritime disputes in the South China Sea with countries including the Philippines, but a UN-backed tribunal has ruled that Beijing's claims were invalid.