AFRICAN UNION and Somali troops are poised to achieve a breakthrough against Al-Qaeda-linked Shebab fighters as they close in on the insurgent base of Afgoye, a top UN official said Friday."Afgoye... controls the exits and entrances to Mogadishu and it has been a military headquarters of the Shebab,” said Augustine Mahiga, the UN special representative for Somalia."Taking Afgoye would make a significant military breakthrough,” he told reporters in the Kenyan capital, warning soldiers to protect civilians as they battled forward into the largest concentration of displaced people in the world.AU and Somali troops were on Friday reported to be within five kilometres (three miles) of Afgoye town, on the fourth day of an assault to wrest the area of the Islamists, who use Afgoye as a launching pad for attacks on Mogadishu.On Thursday, Somali military commander General Abdulahi Osman said the advancing troops were just a "few kilometers” from Afgoye, and expected to take the town "very soon.” The force has continued to advance since then.The extremist Shebab have claimed to have repulsed the attack and have boasted of killing several soldiers, but Somali commanders say the gunmen have fled ahead of their troops and have encountered little heavy resistance.More than 400,000 people, around one third of all the displaced people in Somalia, were living in the Afgoye corridor at the start of the year, fleeing war or drought, according to the UN, which has warned civilians must be protected."Afgoye is both a military and humanitarian imperative but... you are faced with a serious military dilemma,” Mahiga added."It is one of the most difficult operations in an area with the highest concentration of displaced people in the world.”Officials hope that the capture of Afgoye -- some 30 kilometres (18 miles) northwest of Mogadishu -- would deny the Shebab a base from which to launch its recent spate of guerrilla attacks on the capital.Many fighters shifted to the area after pulling out of fixed positions in Mogadishu last August and launching a campaign suicide and grenade attacks.