Insurgents fired on the Somali president's plane today and traded mortar and artillery fire with African peacekeepers in the capital Mogadishu, killing at least 21 and wounding dozen others.<br/><br/> Mortar shells came crashing down on a busy market area in Mogadishu in one of the worst such incidents in months, shattering a relative lull in the Horn of Africa country’s civil war.<br/><br/> According to witnesses, the clashes started when insurgent fighters opened mortar fire on the airport as President Sharif Sheikh Ahmed was flying out of the country for a diplomatic visit.<br/><br/> Sharif was heading to Uganda for an African Union summit on refugees and internally-displaced people and boarding the plane when a hail of mortar shells rained down on the area, police officer Colonel Ali Abdullahi said.<br/><br/>