A Taipei court has sentenced eight Taiwanese soldiers to prison for spying on behalf of China in exchange for money.

The court investigation found that retired military personnel bribed active-duty servicemen with as much as 700,000 Taiwanese dollars (US$21,900; £16,700) to join an espionage network.

One of the men, who played a key role in recruiting soldiers, was given a 13-year prison sentence, the longest of the group.

One of the recruits, a lieutenant colonel, was sentenced to nine years in prison for planning to defect to China by helicopter. Another made an instructional video on how to surrender to China in the event of war.

China considers Taiwan, a self-governing country, as a breakaway province that will eventually come under its control. China does not rule out the use of force to take the island.

The two sides have been spying on each other since the end of the Chinese Civil War in 1949.

Last year, a total of ten people were charged with espionage, and eight were convicted on Thursday. The court acquitted one, while another, a retired officer, remains at large.

“They were seduced by money,” the court said.

“Their actions violated their official duties to be loyal to the country and defend the country and its people. They seriously endangered national security and the well-being of the people of Taiwan.”

Taiwan has recently brought Beijing’s increasing espionage activities to the forefront, and Thursday’s ruling is the latest in a series of cases.

Last month, a sergeant working at a Marine training center was charged with allegedly photographing and leaking confidential defense data to China.

Taipei is also concerned about the increasing frequency of Chinese fighter jet flights around the island.

On Friday, Taiwan President William Lai said the island’s people should “unite as one” and “defend national sovereignty and safeguard democracy.”

He was speaking at an event commemorating the 66th anniversary of China’s attack on Taiwan’s Kinmen Islands.