Japan-China row over the Uotsuri, Minamikojima and Kitakojima islands group of islands in the East China Sea erupted again on Thursday after Japanese Maritime Self Defence Forces reported the presence of Chinese and Russian naval vessels near the islands.
China rebutted a warning from Tokyo, hours after Japan expressed a strong vow to protect its territory following a Chinese naval vessel reportedly sailed close to the disputed islands.
Japan's defence ministry said a Japanese Maritime Self Defence Force guided-missile destroyer, Setogiri, had confirmed the presence of a Chinese ship in a protected zone northeast of Kuba island, part of the disputed Senkakus, which remained there for two-and-a-half hours.
Three Russian battleships also came provocatively close to what Japan considers its territory around that time, the ministry said.
The Russian ships entered the same area around 9:50 pm on Wednesday and left around 3:05 am on Thursday, according to Jiji Press, which added that Russian naval ships had entered the waters on previous occasions.
''We're investigating and analysing whether the two incidents are related,'' said Suga.
Known as Senkaku in Japanese and as Diaoyu in China, the uninhabited islands in the East China Sea are claimed by both countries.
The Diaoyu islands are Chinese territory, asserted the Chinese defence ministry, adding that China's navy has every right to sail through those waters.
Japanese vice foreign minister Akitaka Saiki summoned China's ambassador, Cheng Yonghua, at 2 am, while the maritime incident in the East China Sea was underway, to express ''serious concern'' and lodge a protest, according to a Japanese foreign ministry statement.
Japan will protect the Senkaku islands ''by any means'' and the Chinese frigate which moved early Thursday just outside Japanese territorial waters is a grave matter and ''behavior unilaterally escalating tensions,'' Chief Cabinet Secretary Yoshihide Suga told reporters in Tokyo.
China, wary of US and Japanese intentions is increasing its presence in the seas around it. China has even created artificial islands in the South China Sea to set up naval and air support operations to bolster it defences.
While Chinese patrol vessels are often spotted close to or have even entered waters which Japan considers its own. Thursday's incident involved a larger vessels and involved closer movements of Russian warships and this has added to the Japanese apprehensions over the issue.
Prime Minister Shinzo Abe is reported to have instructed his government to coordinate with the United States and other countries about the incident.
Japan has previously warned that any foreign naval vessels entering into its claimed waters for any reason other than ''innocent passage'' would be instructed to depart by Japanese patrols.
The uninhabited Senkaku islands,northeast of Taiwan, are claimed by Beijing and Taipei, but controlled by Tokyo.
The islands and rocks are in a strategically significant position in shipping lanes, as well as desirable fishing grounds and are near potential oil and gas reserves.
The United States would be compelled to come to the aid of the Japanese, under a mutual security pact, should the disputed islands come under attack, and the Obama administration has confirmed this.