Kano, Nigeria - Boko Haram has claimed a massive attack feared to be the worst in its six-year insurgency and threatened Nigeria's neighbours, as talks began for a regional response to the militants and fears grew of further violence.The confirmation from Boko Haram leader Abubakar Shekau came as no surprise after multiple eye-witness accounts of the attack on Baga, which is thought to have killed hundreds, if not more.But Shekau claimed that the 3 January attack, in which large parts of Baga were burnt to the ground and at least 16 surrounding settlements were razed, was only a prelude to further attacks."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, which was posted on YouTube.He added: "We will not stop. This is not much. You'll see."Heavy clashesThere has been mounting global outrage at the extent of the slaughter, with residents who managed to escape recounting how bodies littered the streets more than two weeks after the initial assault.One civilian vigilante, who fled after hiding for three days, told AFP that he was "stepping on bodies" for 5km as he escaped through the bush.Hundreds of women and children were said to be still being held by the militants at a school and the home of a local lawmaker.At least 20 000 people are said to have fled, many of them across the border into Niger and Chad, heaping pressure on the local authorities there, who fear they could soon be targeted."Heavy clashes" between Islamist fighters and Cameroon soldiers were reported in the far northern border village of Bonderi on Tuesday night in the latest in a series of confrontations.Chad's president Idriss Deby has sent a contingent of troops to help Cameroon repel the threat and has talked of recapturing Baga.I'm readyNigeria and its neighbours met on Tuesday in Niger's capital Niamey to seek a greater regional response to Boko Haram.Ghana's president John Dramani Mahama has also suggested a new force, possibly under the auspices of the African Union, to crush the group.But Shekau dismissed the threat of a wider response and showed off a huge arsenal of weapons, apparently taken from a military base in Doron Baga and used by troops from Nigeria, Niger and Chad."The kings of Africa, you are late. I challenge you to attack me even now. I'm ready," he said.Shekau claimed that Cameroon's president Paul Biya had asked for help because he was "gripped by fear" and mocked Deby for his offer of assistance.Niger's president, Mahamadou Issoufou, was also warned for commiserating with France after the recent Islamist militant attacks against the satirical magazine Charlie Hebdo."Muhammad Yusuf [Mahamadou Issoufou], is that your job? Ah, ah, ah! Muhammad Yusuf, you will see. President of Niger, you will see," he said.Flag burntThe video came after testimony from people fleeing Baga that four villages about 40km from the town had been visited by Islamist fighters and their residents told to leave.Security analysts have said the capture of Baga puts the militants in a strategic position to push south towards Maiduguri, the state capital of Borno State, where the group was founded in 2002.Boko Haram was forced out of Maiduguri in 2013 after the declaration of emergency rule but has in the last six months captured dozens of towns in the far northeast, effectively encircling the city.It has been feared that they want to recapture Maiduguri to form the centre of the hardline Islamic state it has been fighting to establish.Shekau, who has previously declared some captured towns part of Boko Haram's caliphate, burnt the green and white Nigerian flag to cheers of "Allahu Akbar" (God is greatest) and celebratory gunfire."This is the replacement of the Nigerian flag," he said, waving the Islamists' black standard.He also said in English: "Nigeria is dead, [the] constitution is dead."