Japan lodged a fresh diplomatic protest with China on Wednesday, accusing the country of again sending its coast guard ships into waters surrounding contested islands in the East China Sea.

The two countries are locked in a long-running dispute over the uninhabited islets and tensions over them have been a frequent irritant between the countries.

Tokyo has lodged at least 32 protests through diplomatic channels since August 5 over what it says have been 29 intrusions.

Those sparked Kenji Kanasugi, foreign ministry chief of Asia-Pacific affairs, to phone Guo Yan, minister at the Chinese embassy in Japan, the Japanese ministry said in a statement.

"Despite Japan's repeated strong protests, the Chinese side has continued to take unilateral actions that raise tensions on the ground, and that is absolutely unacceptable," Kanasugi told Guo, according to the foreign ministry statement.

Kanasugi also called the intrusions a "violation of Japan's sovereignty" and are "unacceptable".

The vessels left after being warned off by the Japan Coast Guard, officials said.

Ships of the two countries regularly play cat and mouse in the waters but Japan says that Chinese activity has suddenly picked up this month, with local media speculating it is related to a secretive annual summer gathering of top Chinese leaders at a seaside resort east of Beijing.

China is also involved in maritime disputes in the South China Sea and it reacted angrily last month to a UN-backed tribunal ruling that its claims over most of the vital trade artery were invalid.

Japan has called on China to adhere to the decision and the two countries have clashed at recent regional summits and high-level meetings over the issue.

Japan's measures over the east China Sea islands have included summoning China's ambassador to the foreign ministry for a diplomatic dressing down as well as lodging protests via its embassy in Beijing.

The Japan Coast Guard on August 8 said it caught sight of 15 Chinese coast guard ships in waters near the islands -- the highest number ever spotted in the area.

Japan also protested in June after it said a Chinese navy frigate sailed close to its territorial waters near the islands for the first time.