MOGADISHU (Reuters) - Two of Somalia s highest-ranking military officials have resigned, state outlets said on Thursday, in moves that could deal a blow to the war-torn Horn of Africa country s efforts to fight Islamist militants. Somalia has been at war since 1991, when clan-based warlords overthrew dictator Siad Barre and then turned on each other. For a decade, the weak U.N.-backed government has also been fighting alongside African Union troops against Al Shabaab, which aims to topple the government and impose its own strict interpretation of Islam.  On Thursday, state-owned Radio Muqdisho said the army s Chief of Defence Forces Ahmed Jimale Gedi had stepped down and Defence Minister Abdirashid Abdullahi Mohamed had submitted his resignation to President Mohamed Abdullahi Farmaajo s cabinet.  The Cabinet appointed General Abdiweli Jama Hussein ... after Ahmed Jimale Gedi resigned on Thursday,  the radio station said, adding that Mohamed s bid was also accepted. Radio Muqdishu did not disclose the reasons behind their decisions. Neither the officials themselves nor government spokespeople were immediately available for comment. Al Qaeda-allied al Shabaab was driven out of the capital Mogadishu in 2010, but their deadly attacks remain one of the main obstacles to stability in the chaotic nation, which lies along one of the world s busiest shipping routes. Somalia s rebuilding efforts have also been hampered by clan rivalries. Rival regions still sometimes take up arms against each other. 