China sentenced a Taiwanese professor to four years in jail for spying, officials said Wednesday, as tensions between Beijing and Taipei deteriorate further. 

It comes a month after Shih Cheng-ping, a former chief economist for Chinese conglomerate Huaxia Group, made a televised "confession" on state media.

The retired National Taiwan Normal University professor was found guilty by a Chinese court on Tuesday, Beijing's Taiwan Affairs Office said at a press briefing.

Shih -- who disappeared after travelling to the mainland in August 2018 -- was one of several Taiwan residents who appeared on a television programme by state broadcaster CCTV in October confessing to spying on the mainland.

CCTV often broadcasts interviews with suspects admitting to crimes, even before they have appeared in court -- a practice that has long been condemned by lawyers and rights organisations as forcing confessions under duress.

Shih said in the interview that he had passed information from a mainland think tank to Taiwan authorities in exchange for money, and that he hoped his experience would serve as a "warning" to others in Taiwan.

On Wednesday, China's Taiwan Affairs Office spokeswoman Zhu Fenglian said a court in Anhui province had sentenced Shih to four years' imprisonment, a year after China first said it was investigating Shih on national security grounds.

China had "fully safeguarded" Shih's rights during the hearing, she said.

Taiwan has been run as a de facto independent nation for the last seven decades but China sees it as part of its territory, to be reclaimed by force if necessary.

Tensions between Beijing and the democratic island have ramped up in recent months.

Chinese fighter jets and bombers have entered Taiwan's air defence zone with increasing frequency in recent months, and high-profile arms sales by the US to Taipei have angered Beijing.

At least two other Taiwanese individuals are currently being held on national security grounds in China, including the scholar Tsai Chin-shu and democracy activist Lee Meng-chu.

Zhu also said China was in the process of compiling a watch list of Taiwan independence supporters, without providing any details.

Taiwan's Mainland Affairs Council (MAC), the island's top China policy body, condemned Shih's sentencing and called the spying charges "trumped up".

"MAC expresses strong dissatisfaction over the Chinese side's vile actions to intentionally stigmatise normal cross-strait academic exchanges and intimidate Taiwanese people, which have seriously damaged relations between the two sides," it said in a statement.