A protest ship from Taiwan briefly joined around a dozen state-owned Chinese vessels in waters near Japan-administered islands on Friday, officials said, as a territorial row rumbled on.

The Taiwanese-flagged ship was spotted late morning 44 kilometres (28 miles) off Uotsurijima, the largest island in a group known as Senkaku in Japan and Diaoyu in China, the Japanese coastguard said.

Japan controls the islands in the East China Sea, although China and Taiwan also claim ownership.

It was the first time since early July that a protest ship from Taiwan had been seen in waters near the island group.

In Taipei, Taiwan's coastguard said in a statement that the vessel, the fishing boat "Ta Han 711", left Keelung port in north Taiwan late Thursday for the islands and headed back to Taiwan before noon.

The boat, escorted by a Taiwan coastguard vessel during the whole trip, was spotted by three Japanese boats although there was no confrontation between the two sides, the statement said.

"The coastguard will protect our people's voluntary activities to defend the Diaoyu islands," it said.

The Taiwanese ship was adorned with banners reading "Protect Diaoyu" and "Get back Diaoyu" in Chinese, the Japanese coastguard said.

Japan's coastguard warned the ship "by speakers and wireless communications not to enter our territorial waters", a spokesman for the coastguard's Okinawa branch said by telephone.

Under international law, territorial waters extend up to 12 nautical miles from a shoreline.

A total of 13 vessels from China's maritime and fisheries authorities were spotted in waters off the disputed islands on Friday.

Four of them moved into the so-called contiguous zone, a band a further 12 nautical miles from territorial waters, off the island of Taishojima, in early afternoon and remained there as of 0600 GMT, the coastguard said.

State-owned Chinese ships have gone in and out of the contiguous zone -- briefly entering territorial waters on Tuesday -- since the row flared up again when Japan nationalised three of the islands on September 11.

Violent anti-Japan demonstrations spread through major cities in China for eight days but practically vanished on Wednesday, reportedly under orders from Chinese authorities.

On July 4, a boat carrying activists and four patrol boats from Taiwan entered territorial waters around the islands.