Taiwan is vetting hundreds of thousands of military service members, public school teachers and civil servants in a bid to root out potential homegrown Chinese sympathizers, as Beijing intensifies espionage on the island.

Alarm is growing in Taiwan over the extent of China’s infiltration on the self-ruled island, which Beijing claims is part of its territory and has threatened to seize by force.

Prosecutors last week charged four recently expelled members of the ruling Democratic Progressive Party — including a former staffer in President Lai Ching-te’s office — for sharing state secrets with Beijing.

While Taipei and Beijing have spied on each other for decades, analysts warn the threat to Taiwan is more serious given the risk of a Chinese attack.

The main targets of Chinese infiltration have been retired and active members of the military, persuaded by money, blackmail or pro-China ideology.

Lai, an outspoken defender of Taiwan’s sovereignty and loathed by Beijing, has branded China a “foreign hostile force” and sought to raise public awareness about Chinese actions he says threaten national security.

After a sharp rise in the number of people prosecuted for spying for China in recent years, the government is trying to identify people within its own departments, military and public schools with a possible allegiance to Beijing.

Anyone on the public service payroll found with Chinese residence or other identification cards risks losing their Taiwanese household registration, effectively their citizenship.

“The reason we started to survey (for Chinese IDs) is because China uses this way to coerce Taiwanese people, to penetrate our system, especially the public service,” DPP lawmaker Wang Ting-yu told Agence France-Presse.

“The threat is getting worse and worse and we have to deal with that.”