(Reuters) -- Two Chinese patrol ships entered Japanese territorial waters near disputed islets claimed by both countries early Friday, Japan's Coast Guard said, marking the latest incident in a long-running dispute between China and Japan.

The Chinese ships have not followed the coast guard's order to leave the territorial waters, a Japan coast guard official said.

Tension flared last month when Japan detained a group of Chinese activists who had landed on the islands, known as Senkaku in Japanese and Diaoyu in Chinese.

Bilateral ties were strained further on Tuesday when Japan, which controls the islands, said it had bought them from a private owner, ignoring warnings from China