This handout picture taken by the Japan Coast Guard on December 17, 2012 shows a Chinese marine surveillance ship cruising near the disputed islands known as Senkaku islands in Japan and Diaoyu islands in China, in the East China Sea.
China sends four coastguard vessels to waters around a chain of disputed islands in the East China Sea as a territorial dispute between Beijing and Tokyo carries on.
The Japanese coastguard said the Chinese ships sailed into the territorial waters of the islands, known as Diaoyus in Chinese and as Senkakus in Japanese, on Monday morning.
It added that the vessels remained near the inhabited islands for about two hours.
The move follows Tokyo’s approval of a plan to shoot down Chinese drones if they fly over Japan's airspace.
China has threatened to take firm countermeasures, if Japan fires on its drones.
Chinese Ministry of National Defense spokesman Geng Yansheng said in a statement issued on Saturday, “If Japan takes willful measures to shoot down China’s planes, we will regard it as severe provocation and an act of war, we will strike back resolutely. All consequences should be borne by the side that causes the trouble.”
“Japan’s belligerent rhetoric is deliberately provocative. Normal training and flights of China’s military planes including unmanned planes in waters of the East China Sea accords with international law and international practices. China’s planes have never infringed airspace of other countries, and China never allows planes of other countries to infringe its airspace,” Geng stated.
Japan’s Prime Minister Shinzo Abe has said that Tokyo is ready to be more assertive towards China.
China and Japan have long been locked in a dispute over the sovereignty of the uninhabited islands in the East China Sea.
Last month, Chinese Foreign Minister Wang Yi said Beijing is ready to hold talks with Japan over the maritime row. Wang also blamed Japan for the rising tensions between the two countries.
Tokyo signed a deal on September 11, 2012 to buy three of the islands from their private Japanese owner in line with plans to nationalize the archipelago.
China maintains that the islands are an inherent part of its territory and it has indisputable sovereignty over them. Japanese government, on the other hand, regards the islands as a part of Ishigaki, Okinawa Prefecture.
The islands have been under Japanese administrative control since the reversion of Okinawa to Japan from US administrative rule in 1972.
Potentially large deposits of natural gas off the islands are believed to be the source of the territorial dispute.