Taiwan’s coast guard detained a cargo ship and its Chinese crew on Tuesday and said it was investigating whether the vessel had deliberately cut an undersea telecoms cable in the latest possible breach of the island’s communication lines.

TAIPEI—Taiwan detained a cargo ship and its eight Chinese crew members after an undersea fiber-optic cable was severed, in a stepped-up effort to police such incidents, which are often seen as part of China’s pressure campaign targeting the self-ruled island.

Taiwan, which China claims as its own territory, has repeatedly complained about "grey zone" Chinese activities around the island, designed to pressure it without direct confrontation, such as balloon overflights and sand dredging.