Yaounda - The head of Boko Haram - the enigmatic Abubakar Shekau - has openly taunted the countries being menaced by the armed Islamist group as they call for a united effort to crush it.In a video claiming responsibility for the bloody attack on the Nigerian town of Baga in early January, Shekau poured contempt on the presidents of Chad, Cameroon and Niger, goading Chad's Idriss Deby Itno with the message: "African kings... I challenge you to attack me now. I am ready."The Baga attack on 3 January in which hundreds are feared to have been killed has been called a crime against humanity by Washington and Paris.In the video that has just emerged on YouTube, Shekau also accused Cameroon President Paul Biya of being too afraid to ask for help in the face of group's ever-increasing belligerence.Cameroon has also seen repeated recent attacks, including the kidnapping of dozens of people, mostly women and children, during a deadly attack on Sunday. To Niger's President Mahamadou Issoufou, who Shekau noted had commiserated with France after the recent Islamist attacks in Paris, he said: "Muhammad Yusuf [Mahamadou Issoufou], is that your job? Ah, ah, ah! Muhammad Yusuf, you will see. President of Niger, you will see," he said.Shekau's provocative video, came as a regional summit opened in Nigeria on Tuesday aimed at stopping Boko Haram whose insurgency has left 13 000 dead and forced 1.5 million from their homes since 2009.Leaders from Ghana and Chad have called for a unified effort in confronting the Islamist militants.Chad sent a convoy of troops and 400 military vehicles on Saturday into neighbouring Cameroon to fight Boko Haram, as Nigeria's neighbours appeared to be losing patience with the Nigerian army's passivity.We will not stopLittle is known of "Shekau", with some experts and Nigerian security officials insisting that he is in fact a composite character whose role is taken by a rotating cast of different militant fighters.According to security services, the original Abubakar Shekau was the son of poor farmers who was radicalised while attending theological schools and took over Boko Haram in 2010.The Nigerian military said last September that a man posing as Shekau in videos posted online had in fact been killed after fighting with troops in the far northeast.The United States and other experts, however, have questioned the credibility of that claim while Shekau outright rejected it in a video obtained by AFP last October."Here I am, alive. I will only die the day Allah takes my breath," he said in that video.The insurgent leader, sanctioned by the UN Security Council and declared a "global terrorist" by the United States, issued a similarly boastful denial in 2013 after the military claimed he may have died from a gunshot wound.Boko Haram's claim of responsibility for the Baga attack - in which large parts of the town were burnt to the ground and at least 16 surrounding settlements razed - was not unexpected given the testimony of survivors."We killed the people of Baga. We indeed killed them, as our Lord instructed us in His Book," Shekau said in the 35-minute message.He added: "We will not stop. This is not much. You'll see."