Formal negotiations between Japan and the Philippines to delimit maritime boundaries have sparked anger in China, leading to coastguard patrols in the contested waters off eastern Taiwan.

The waters off eastern Taiwan have become a new flashpoint after Tokyo and Manila announced formal negotiations to delimit the maritime boundary of the exclusive economic zones (EEZs) and continental shelf between Japan and the Philippines. On Monday, China’s coastguard carried out enforcement patrols in the region, exercising Beijing’s claimed right to maintain its own EEZ and continental shelf rights under both domestic and international law. The Taiwanese coastguard said it shadowed and...