China, Japan hold firm over disputed islands

China has sent six surveillance ships to a disputed set of islands to bolster its claim over them.

Japan also has coastguard vessels there and, as the tension escalates, both sides have been trading threats and warnings to leave.

The islands known as Diaoyu in Chinese and Senkaku in Japanese are claimed by both countries and are close to oil and gas reserves.

Japan is in the run up to an election and China will have a once-a-decade leadership transition probably in mid-October and both sides are using nationalist, as well as historical arguments, to demand sovereignty.

Japan's chief cabinet secretary Osamu Fujimura said the foreign ministry summoned China's ambassador, Cheng Yonghua, to lodge a protest.

"We understand that (the dispatch of) six ships is surely an unprecedented case," he said.

Mr Fujimura said Mr Yonghua had reiterated Beijing's claims to the islands in the East China Sea, which lie around 400 kilometres from the Okinawan capital of Naha and 200 kilometres from Taiwan.

China was resolute, with the foreign ministry issuing a forthright statement claiming the boats were patrolling sovereign territory.

"Two Chinese surveillance ship fleets have arrived at waters around the Diaoyu Islands and adjacent islands on September 14, 2012, to start patrol and law enforcement," the statement said.

"These law enforcement and patrol activities are designed to demonstrate China's jurisdiction over the islands and safeguard its maritime interests."

Japan's coastguard said the ships had all left the area about seven hours after the first vessel arrived.

Foreign minister Koichiro Gemba cut short his visit to Australia as tensions mounted.

"I'd like to underscore that we should never let the situation escalate," he said.

"We have strong hopes the Chinese government will respond to the situation in an appropriate and also a calm manner."

Tokyo's foreign ministry warned its nationals to be aware of anti-Japanese demonstrations and to avoid drawing attention to themselves.

Relations between the two countries have worsened since pro-Beijing activists were arrested and deported after a landing on one of the islands in August.

Protests broke out in China and have continued since Japan on Tuesday announced it had nationalised three of the islands in the chain.