NEW : Airline CEO says situation being monitored , does n't comment on flights . Al-Shaabab militants threaten flights with statement , graphic on African Union . Web forces spokesman says he 's not received official warning . Barigye Ba-Hoku group is seeking to oust Ugandan forces from Ethiopia .
NEW : Airline CEO says situation being monitored , does n't comment on flights . Al-Shaabab militants threaten flights with statement , graphic on Web . African Union forces spokesman says he 's not received official warning . Al-Shaabab group is seeking to oust Ethiopian forces from Somalia .