BEIJING - China warned of tough countermeasures after a Japanese court on Sunday extended the detention of a Chinese trawler captain, illegally held in Japan after collisions in waters off China's Diaoyu Islands on Sept 7.

"China demands that Japan immediately release the captain without any preconditions," Foreign Ministry spokesman Ma Zhaoxu said in a press release, repeating that China viewed the detention as illegal and invalid.

"If Japan clings obstinately to its own course and insists on making one mistake after another, China will take strong countermeasures and all the consequences should be born by the Japanese side," he said.

A Xinhua News Agency report also said Vice-Foreign Minister Wang Guangya "made solemn representations" to Japanese Ambassador Uichiro Niwa on Sunday evening to express strong indignation and protest over Japan's prolonged detention.

Tensions between the two countries have escalated since Japan illegally arrested Chinese trawler captain Zhan Qixiong, accusing him of "obstructing public officers" when his boat was intercepted by Japan Coast Guard patrol vessels.

Japan's decision (on detention extension) has "seriously damaged Sino-Japanese bilateral exchanges", according to Xinhua, citing a separate response from the Foreign Ministry.

China has suspended ministerial and provincial-level bilateral exchanges with Japan, halted talks on increasing flights between the two countries and postponed a meeting about coal with Japan, the report said. "The scale of Chinese tourists visiting Japan has been seriously affected," it added.

Japan's Kyodo news agency reported that the trawler captain's detention had been extended until Sept 29. This was the second time an extension was granted. A Japanese court first approved an extension on Sept 10. Prosecutors can hold Zhan for a total of 20 days before deciding whether to take "legal action".

China has repeatedly demanded Japan free the captain and, in protest, has cancelled scheduled talks with Japan over natural gas reserves in the East China Sea.

China has also sent marine surveillance ships to strengthen law-enforcement activities in "relevant waters to safeguard China's maritime rights and interests", Foreign Ministry spokeswoman Jiang Yu said over the weekend. "Law-enforcement ships have also been dispatched to cruise in the area and protect fishermen and their boats."

Asked to confirm reports that China has transported materials to Chunxiao oil and gas field and may carry out maintenance operations there, Jiang said China possesses full sovereign and jurisdictional rights over Chunxiao and China's activities there are completely reasonable and lawful.

The Nikkei Business Daily reported earlier on Sunday that Japan may start drilling near the gas field in the East China Sea if China does the same.

Zhou Yongsheng, professor of Japan studies at the China Foreign Affairs University, said the chances of relations between the two countries getting back on track in the short term were slim.

"When we handled territorial disputes with Japan in the past, we always put China-Japan friendship at the highest priority. However, the DPJ (the ruling Democratic Party of Japan) administration does not have an effective strategy to support the development of the China-Japan relationship. Instead, it has been led by the media," Zhou said.

He noted that Japan did not cherish the achievements of China-Japan relations.

Japan's decision to extend the illegal detention of the Chinese captain aims to strengthen its claim of sovereignty over the Diaoyu Islands, he said. "China should take strong countermeasures."

In this regard, he added, the Chinese government had more cards in hand than the Japanese as their economy is largely dependent on China.

Shen Shishun, an analyst at the China Foundation of International Studies, said that Japan is still trying to push the serious situation in relations to a deadlock despite repeated protests from China.

Shen urged Japan to stop its current way of handling disputes.

"China is not willing to see more damage done to regional peace and bilateral cooperation," he said. "The final solution will be found through diplomatic negotiations."