North Korea has sent about 300 security agents to China in a bid to strengthen monitoring on the North's overseas workers following a high-profile defection by over a dozen North Korean restaurant staffers, an activist claimed Thursday.

In late June, the North dispatched security agents in their 30s and 40s to the neighboring country in an effort to step up supervision on North Korean workers, according to North Korean defector Kim Yong-hwa, head of the NK Refugees Human Rights Associations of Korea.

"The agents were dispatched to North Korea-run restaurants, Chinese companies and trade missions where North Koreans are working," Kim told Yonhap News Agency, citing a source familiar with the situation.

"They are closely checking what North Korean workers say over the phone. Agents do not allow them to go out freely as well," he said. "North Koreans working in China would face difficulty in escaping their workplaces."

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

Overseas restaurants operated by North Korea 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 in overseas restaurants are among the 50,000 workers sent abroad by the regime to earn much-needed hard currency to help it tackle economic hardship amid the U.N. sanctions on the North.

North Korea handpicks workers who are loyal to the regime and sends them overseas to work at restaurants.

Kim said that North Korean leader Kim Jong-un orders the dispatch of security agents and rotates them every three months to prevent them from colluding with North Korean officials they are supposed to monitor. (Yonhap)