Japanese Prime Minister Shinzo Abe has reiterated Tokyo's hard-line position on the Diaoyu Islands, during a tour of Okinawa, amid continuing tension over the East China Sea islands.

Observers said a speech by Abe on Saturday represented a setback.

Abe was commenting on patrols by Chinese government vessels in the waters off the islands, while praising officers of Japan's regional coast guard office on the southern island of Okinawa.

The Japanese prime minister predicted that the tension over the islands "to continue" and that the Japanese government will support the coast guard with more advanced patrol vessels.

In an earlier speech, addressing personnel at a Japanese airbase in Naha, Abe said the security environment surrounding the country is "increasingly tense", and he will "resolutely protect" Japan's territory, Japan's Jiji Press News Agency reported.

Fighter jets have been scrambled in recent months from Japanese Self-Defense Force bases, to confront Chinese aircraft on regular patrol missions above the East China Sea, increasing tension further.

However, Abe's remarks came just a day after a speech to Japan's National Diet, which had aimed at improving soured ties with China, strained since last September's illegal "purchase" of China's Diaoyu Islands by Japan.

Zhou Yongsheng, a professor on Japanese studies at China Foreign Affairs University, said that Abe's comments in recent days had offered "contradictory postures", mixing some friendly signals to Beijing while at the same time maintaining his persistent right-wing stance.

Abe and his key cabinet members have repeatedly vowed to ensure a "mutually beneficial relationship" with China, a key political consensus first reached by both sides in 2008.

Zhang Tuosheng, a researcher at the China Foundation for International and Strategic Studies, added that the possibility for major conflict over the islands remains "relatively low" as both sides have been trying to avoid a direct clash.

China on Saturday commissioned the Haijian 8002, the country's first kiloton-class marine surveillance ship which can travel up to 9,260 km without refueling.

It is one of 36 ships that the China Marine Surveillance, under the State Oceanic Administration, plans to build to use for missions guarding its sovereignty, China Central Television reported.

Tension in the East China Sea has triggered worries in Washington, too, prompting the Maritime Administration of the US Department of Transportation to issue an advisory to all US vessels on Jan 28 to "steer clear of any activity" in the waters off the islands.

Gao Haikuan, vice-president of the China Society of the History of Sino-Japanese Relations, said that Japan should avoid fanning any confrontation, and continue in pursuit of a "strategic and mutually beneficial relationship" with China.

"Confrontation is not expected between partners which have reciprocal economic interests," Gao added.