Nairobi - Somalia's Shabaab fighters have warned they will "cut the throat" of members who shift allegiance from al-Qaeda to Islamic State (ISIS), amid reports some factions have already been punished for doing so."If anyone says he belongs to another Islamic movement, kill him on the spot," top Shabaab official Abu Abdalla, said in a radio broadcast on Monday. "We will cut the throat of any one... if they undermine unity."The Shabaab, East Africa's long-time al-Qaeda branch, is fighting to overthrow the internationally-backed government in Mogadishu, which is protected by 22 000 African Union troops.The Islamists have lost much ground in recent years but remain a threat in both Somalia and neighbouring Kenya, where they have carried out a series of attacks.The reported divisions come at a time when Islamic State in Iraq and Syria has become the jihadist franchise of choice, attracting fighters from abroad and the allegiance of other militant groups such as Boko Haram in Nigeria.But al-Qaeda has also recently expanded its territory in Yemen, just across the Gulf of Aden from Somalia.A handful of Shabaab factions have switched allegiance to ISIS but the shift has failed to gain momentum. Pro-ISIS groups have been attacked and their leaders assassinated as Shabaab emir and al-Qaeda loyalist Ahmed Diriye seeks to shore up his control.Last month President Hassan Sheikh Mohamud said "the now-public dispute" within Shabaab showed the group had "lost its way".But Abdalla said Shabaab remained united. "The world wanted us to be divided," he said."This is a collective decision, and anybody who wants to join another Islamic group must leave the country to meet them where they are." he said. "I swear by the name of God we will not tolerate the acts of saboteurs."