North Korea has sent about 800 security agents to areas bordering China in a bid to beef up monitoring on the North's foreign workers and prevent them from fleeing their posts, a source familiar with the move said Monday.

China's authorities have raised their guard against North Korea's move to dispatch hundreds of agents to Chinese cities, including Dandong and Yenji, where tens of thousands of North Koreans are working, according to the source.

The North Korean agents, who belong to the Ministry of State Security or the North's reconnaissance bureau tasked with carrying out espionage missions, are intensifying their monitoring of workers employed at North Korea-run restaurants and Chinese companies, the observer claimed.

In April, 13 North Koreans working at a Pyongyang-run restaurant in the Chinese eastern port city of Ningbo defected to South Korea en masse. In June, three more North Korean restaurant employees working in China escaped to Seoul.

Restaurants operated by North Korea in foreign countries have served as one of the main sources of hard currency for North Korea.

The North is suspected of using the money to bankroll its nuclear and missile programs.

North Koreans employed by foreign restaurants are among the 50,000 workers sent abroad by the regime to earn much-needed hard currency to help it tackle economic hardships amid the U.N. sanctions on the North.

Another North Korean watcher said that the North's leader Kim Jong-un may be seeking to kidnap South Korean activists in China believed to be helping North Korean defectors.

South Korea's foreign ministry earlier said it is advisable for South Koreans not to visit Chinese border areas with North Korea due to security concerns. (Yonhap)