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China's most senior diplomat has called on Japan to immediately release the captain and 14 crew of a fishing boat seized in disputed waters last week.

State Councillor Dai Bingguo warned Tokyo to make a "wise political decision" over the matter.

On Friday, China postponed a session of planned diplomatic talks after a Japanese court ruled the boat's captain could be detained for another 10 days.

Prosecutors say his vessel deliberately rammed Japanese patrol boats.

The area where the Chinese trawler was seized on Tuesday is close to uninhabited islands, known as Senkaku in Japan and Diaoyu in China, which are controlled by Japan but also claimed by China and Taiwan.

Closer to Taiwan, the islands and Taiwan itself were ceded to Japan by the Manchu emperor in 1895. After Japan's defeat in World War II, Taiwan was returned to the Kuomintang government, but not the islands.

On Sunday, Mr Dai increased pressure on Japan by summoning its ambassador, Uichiro Niwa, to again demand the release of the fishermen.

It was the fourth time that the Japanese envoy had been called to the foreign ministry over the incident, but correspondents say it is highly unusual for someone as senior as the state councillor to intercede.

Mr Dai advises China's leaders on foreign policy and serves on the State Council, outranking the foreign minister within the Communist Party.

"[He] solemnly stated the Chinese government's major concerns and urged Japan not to misjudge the circumstances and to make the wise political decision of immediately returning the Chinese fishermen and their boat," the official Xinhua news agency reported.

Meanwhile, Japanese coast guard officials took the trawler and its crew out to sea off the southern island of Okinawa to test its capabilities and recreate the circumstances of the collision to help investigators.

Officials are also reportedly trying to ascertain if the boat illegally entered waters which Japan considers its territory.

The Chinese foreign ministry said it firmly opposed any form of investigation by the Japanese authorities into the incident.

"Japan's so-called gathering of evidence is illegal, ineffective and futile," spokeswoman Jiang Yu said. "China urges Japan to stop actions that escalate the situation."

But Mr Niwa gave no indication that Japan was preparing to back down.

"We have maintained the position that we will solemnly handle the case in strict accordance with domestic law", he said, according to the Kyodo news agency.

A Japanese court has allowed prosecutors to keep the captain, Zhan Qixiong, in custody until 19 September before deciding whether to press charges. His crew have been told they are free to return to China.