China claims self-ruled, democratic Taiwan as part of its territory to be seized one day, by force if necessary.

The two sides have been spying on each other since splitting after a civil war between Chinese nationalists and communists in 1949.

Taiwan High Court's branch in southern Kaohsiung City said Wednesday that the accused -- identified by his surname Liu -- was found guilty of violating the National Security Act for "developing an organisation" for China.

He was also convicted on several counts of violating the Classified National Security Information Protection Act for handing over confidential national security information to Beijing, which is punishable by a maximum seven-year jail term.

Liu received a combined sentence of 20 years for his offences, which the court did not provide details as the case concerns national secrets, it said in a statement.

Prosecutors had launched an investigation into Liu in January on suspicions of recruiting at least six active-duty officers in the air force and navy over a period of eight years on behalf of Beijing.

Five officers involved in the case received prison terms -- ranging from six months to 20 years -- while one was acquitted, the court said.

Liu was recruited by Beijing during a business trip to the mainland in 2013, and had developed a network of informants, whom he paid through shell companies, according to local media.

A number of former high-ranking Taiwan military officials have in recent years been accused of developing spying networks for China.

A retired air force major general received a four-year suspended sentence in January for accepting meals and trips offered by a Hong Kong businessman allegedly acting on behalf of Beijing.