China instituted a silent alarm system in all houses in a town near the North Korean border to try to arrest defectors, two ethnic Koreans in China said Friday. One said he saw the device, which could transmit conversations, during a visit to a relative in Yanbian, a Korean Autonomous Prefecture in northeastern China, Yonhap reported. The South Korean news agency said China recently detained dozens of North Korean defectors, reportedly returning some to their homeland where they could face harsh punishment despite international appeals. The Korean said the surveillance system allows Chinese residents to send a signal to police surreptitiously if North Koreans come to their houses asking for help after crossing the China-North Korea border. The other Korean, a military officer in Yanbian, said Chinese authorities indicated they would expand the silent alarm system into other areas along its porous border with North Korea, Yonhap said. North Koreans have crossed into China\'s northeastern areas to escape chronic food shortages and political oppression, observers say.