A South Korean humanitarian agency says a mysterious epidemic has been spreading along North Korean towns along the border with China, killing dozens of children.

The Seoul-based Good Friends agency says five or six children have died every day since the highly infectious disease emerged on April 27.

The group says North Korean doctors have been unable to diagnose or cure the disease, and have said it could be avian flu or the hand-foot-and-mouth disease which has killed dozens of children in China in recent weeks.

Good Friends reports the outbreak has mainly affected state-run daycare centres and kindergartens, with patients showing flu-like symptoms before succumbing.