Burundi and Ugandan forces make up the bulk of the AU mission in Somalia . 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 . Comment:
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 . Comment: