China on Friday sent four coastguard vessels into waters near a group of islands also claimed by Japan, the state-owned Xinhua news agency reported.
The Japanese-administered Senkaku Islands are also claimed by China and Taiwan, which call them Diaoyu and Tiaoyutai respectively.
The move is the latest in a long-running dispute over the territory, which encompasses busy shipping lanes and is thought to be rich in fish stocks and mineral resources.
Japan’s purchase of three of the islets in September sparked protests in dozens of Chinese cities and led to a boycott of Japanese products in the world’s second-largest economy.