Seven North Koreans from two families defected to an unidentified country despite the communist country's tight security for its founder's birthday, a U.S. radio outlet said Wednesday.

Their defections occurred during the special alert period set from April 1 to 20 by the North Korean regime to commemorate the April 15 birthday of Kim Il-sung, the Washington-based Radio Free Asia reported.

The birthday known as the Day of the Sun is a major holiday in the isolated country.

Quoting a North Korean source, the RFA said the seven residents in Musan, North Hamgyong Province, disappeared overnight on Friday amid heightened security, with the roads and the mountain passes in the border area with China being placed under tight control.

The RFA said the North's state security ministry immediately launched an investigation into their whereabouts but have been unable to locate them.

The source said North Korean residents nearby were surprised by the escape.

Normally, the North mobilizes border garrison units, worker peasant red guard members and militia members when security is heightened.

Other sources, however, said that although the North is in a state of high alert readiness, the overall security network is often lax during holidays because state agents and border guards are sometimes drunk.

In the past, the North designated a "special guard period" on such national holidays as New Year's Day, Kim Jong-il's birthday on Feb. 16, Kim Il-sung's birthday on April 15, National Foundation Day on Sept. 9 and the Workers' Party's founding day on Oct. 10. (Yonhap)