HONG KONG (REUTERS) - Hong Kong blocked a group of Chinese activists from embarking upon what was believed to be a voyage to disputed islands in the East China Sea on Wednesday, preventing a diplomatic run in with Japan.

The 15 Hong Kong and Chinese activists and crew loaded food, protest banners and other supplies onto their fishing vessel, Kai Fung No 2, on Wednesday claiming they were going "fishing" in the Spratly islands straddling the strategic sea lanes of the South China Sea.

Marine Department officials inspecting their vessel, however, read out a notice warning them to only operate in Hong Kong waters while citing news reports suggesting they were in fact headed for the rocky, uninhabited islands claimed by China and Japan in the East China Sea. The islands are called the Diaoyu in Chinese and the Senkaku by Japan.

Last August a group of activists from Hong Kong eluded Japanese coastguard vessels to land on the disputed islets.

The activists waved Taiwanese and Chinese flags upon landing before being detained by Japanese authorities for several days, triggering a diplomatic spat at a time of tension between Beijing and Tokyo.

Ties between Asia's two largest economies deteriorated sharply after Japan bought three of the disputed East China Sea islets from a private owner in September 2012, sparking protests and boycotts of Japanese goods across China.