Taiwan's military vowed on Wednesday to step up counter-espionage efforts as authorities investigated several serving and former military officers suspected of spying for China. China, which is pressing the island to accept its sovereignty, has in recent years mounted a sustained espionage campaign to undermine democratically governed Taiwan's military and civilian leadership, a Reuters investigation has found. A lieutenant colonel surnamed Hsiao, based in the army's Aviation and Special Forces Command, had been detained on suspicion of leaking defence secrets to "foreign forces including China" and "developing organisations" in Taiwan, the official Central News Agency (CNA) reported.