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.

Investigators searched the Command headquarters in the northern city of Taoyuan this week, the CNA reported, adding that four retired military officers as well as a "middleman" surnamed Hsiao were also being investigated.

The defence ministry said in a statement authorities had gathered "concrete evidence" of illegal activities.

"Facing infiltration by the Chinese Communist Party, the national forces will continue to boost counter-espionage education and raise awareness," the ministry said, adding it was saddened by the crime of "selling out the country and people".

Responding to a question about the reports at a press conference, Deputy Secretary-General to the Presidential Office Alex Huang said the incident was "shameless" and called for thorough investigations.

"Betraying your own fellow soldiers and country should be punished by law strictly," he said, adding that authorities had been working hard to prevent such incidents happening again.

China's Taiwan Affairs Office did not immediately respond to a request for comment.

China, which views Taiwan as its own territory, has stepped up military and political pressure over recent years to try to force the island to accept its sovereignty, which the government in Taipei rejects.

In the past decade or so, at least 21 serving or retired Taiwanese officers with the rank of captain or above have been convicted of spying for China, according to a Reuters review of court records and reports from Taiwan's official news agencies.