The soldier, identified only by his family name Chen, was reported missing on Thursday after morning roll-call on Erdan Island, which lies just five kilometres (three miles) off the mainland coast.

"It has been confirmed that he is in mainland China," said Chiu Tai-san, head of the Mainland Affairs Council, Taiwan's top policy-making body on China.

When asked if Chen had deserted his post, Chiu said the government will "follow up on the situation".

"The defence ministry has relevant mechanisms for identifying deserters," he added.

The defence ministry, which formed a special task force to search for the soldier, has so far not commented on his presence in the mainland.

Defence Minister Chiu Kuo-cheng said Friday that it was too early to conclude if the soldier was "running away".

Chen is a staff member in a kitchen at the Lieyu Garrison Battalion on Erdan, according to the Taiwanese military.

Erdan is one of the Kinmen islands, which lie close to the Chinese port city of Xiamen.

The soldier's disappearance comes at a time of heightened tensions between China and self-ruled Taiwan, which Beijing considers its territory and has vowed to take one day, by force if necessary.

Chinese sabre-rattling towards Taiwan has become more pronounced in recent years.

A visit to Taiwan in 2022 by then US House Speaker Nancy Pelosi prompted a furious China to hold its biggest military drills around the island in years.

And China's ruling Communist Party for the first time enshrined its opposition to Taiwanese independence in its constitution in October last year.