ARMED militants kidnapped five Koreans and killed ten security-force members in an attack on a small natural-gas plant in Nigeria's southern delta yesterday, authorities and security sources said. The Movement for the Emancipation of the Niger Delta (MEND) claimed responsibility, and said the Koreans would be freed in exchange for a jailed militia leader who is on trial for treason and was denied bail by a Nigerian court on Tuesday. The attack on the plant operated by the oil giant, Royal Dutch Shell, comes three days after eight foreign oil workers were released by a different group of kidnappers, and is the latest sign of a rising militancy hitting output for Africa's top oil producer.