Beijing claims self-ruled Taiwan as its territory and has ramped up military and political pressures on the island in recent years.

The sergeant, surnamed Chen, worked at a navy training centre and was recruited by an unspecified number of people who "collected intelligence for mainland China" via messaging apps in 2022, said Taiwan High Prosecutors' Office.

"Between April 2022 and February 2023, he photographed secret national defence information in (the counties of) Pingtung, Yilan and other places with his mobile phone," the office said in a statement.

"(He) sent the information four times via Line and Telegram... for a total illegal gain of NT$170,000 ($5,230)."

The office said Chen was charged with violating the Criminal Code of the Armed Forces and the Anti-corruption Act.

When asked to comment on the indictment, defence ministry spokesman Sun Li-fang said the military had worked with national security units on the case.

"In recent years, the CCP's (Chinese Communist Party's) infiltration has indeed posed a very serious threat to the military. The threat is no less than... threats posed by missiles or aircraft and ships," he told reporters.

China maintains a near-daily presence of warships, drones and fighter jets around Taiwan, and earlier this year had launched war games following the May 20 inauguration of Taiwanese President Lai Ching-te.

Beijing regards him as a "dangerous separatist" due to his defence of Taiwan's sovereignty.

In pre-recorded footage for a military television programme that aired Tuesday, Lai warned soldiers at an air force base in central Taichung city to be vigilant.

"China's infiltration and spying will not stop," he said, dressed in military fatigues.

"You must always be vigilant, pay attention to your own information security, and do not fall into traps."

The sergeant's indictment was the latest in a recent string of spying cases.

In April, a father and son duo were jailed for eight years for collecting confidential military information and trying to develop a spying "organisation" for Beijing.