* Chinese ships leave waters in afternoon
* Japan lodges official protest
* Bad weather might delay arrival of Taiwan ships
* U.S., Japan forces hold joint exercise in Guam
By Kiyoshi Takenaka and Antoni Slodkowski
TOKYO, Sept 24 (Reuters) - Four Chinese ships briefly entered what Japan considers its territorial waters near disputed islands in the East China Sea on Monday, prompting an official protest from Tokyo and renewed diplomatic efforts to cool tensions between the rivals.
In a move that could further complicate the territorial row that is threatening relations between Asia's biggest economies, a group of fishermen from Taiwan -- which also claims the rocky isles -- said as many as 100 boats escorted by 10 Taiwan Coast Guard vessels were headed for the area.
China's Xinhua news agency said in the morning that two civilian surveillance ships were undertaking a "rights defence" patrol near the islands, citing the State Oceanic Administration, which controls the ships. Two fishery patrol vessels were also detected inside waters claimed by Japan.
Japan lodged an official protest.
By afternoon, all four Chinese vessels had moved further away, the Japanese Coast Guard said.
Sino-Japanese relations deteriorated sharply after Japan bought the islands, called Senkaku in Japan and Diaoyu in China, earlier this month, sparking anti-Japan protests across China.
"In recent days, Japan has constantly provoked incidents concerning the Diaoyu islands issue, gravely violating China's territorial sovereignty," Xinhua reported.
The ship patrols were intended to exercise China's "administrative jurisdiction" over the islands, it said.
"Following the relevant laws of the People's Republic of China, (the ships) again carried out a regular rights defence patrol in our territorial waters around the Diaoyu islands."
Sino-Japanese ties have long been plagued by China's memories of Japan's military aggression in the 1930s and 1940s and present rivalry over regional influence and resources.
Japanese Vice Foreign Minister Chikao Kawai will visit China on Monday to discuss Sino-Japanese relations with Chinese Vice Foreign Minister Zhang Zhijun, the Foreign Ministry said.
The arrival of Taiwan vessels in the area could complicate the potentially fraught game of cat-and-mouse being played near the islands, where mainland China has launched an effort to assert sovereignty by sending government ships into the disputed waters.
Taiwan television showed boats bound for the islands leaving Suao port in heavy rain, sporting banners and large Taiwan flags. News reports said bad weather could delay their arrival, expected overnight on Monday.
The Taiwan fishing group said their boats would sail around the islands to reassert their right to fish there and did not rule out trying to land on the rocky isles.
Taiwan Defence Minister Kao Hua-chu told parliament that the military was ready for any contingency, but did not elaborate.
Taiwan has traditionally had friendly ties with Japan, but the two countries have long squabbled over fishing rights in the area. Beijing deems Taiwan to be an illegitimate breakaway province, and the two sides both argue they have inherited China's historic sovereignty over the islands, which are near rich fishing grounds and potentially huge oil and gas reserves.
The latest flare-up in tensions over the islands comes at a time when both China and Japan confront domestic political pressures. Japanese Prime Minister Yoshihiko Noda's government faces an election in months, adding pressure on him not to look weak on China.
China's Communist Party is preoccupied with a leadership turnover, with President Hu Jintao due to step down as party leader at a congress that could open as soon as next month.
On Monday, Noda told reporters before leaving for New York to address the U.N. General Assembly that he would stress the importance of solving problems between nations by the rule of law, but that he had no plan now to mention specific issues in his speech to the world body.
Some 40 Japanese troops and 2,200 U.S. Marines, meanwhile, are conducting a joint drill on and around the U.S. territory of Guam aimed at improving their ability to defend remote islands, Japan's defence ministry said. The exercise runs from Aug. 21 through Sept. 26.
Worries are simmering that the row could hurt the economic ties that closely bind China and Japan. China is Japan's largest trading partner. In 2011, their bilateral trade grew 14.3 percent in value to a record $345 billion.