Dakar - The four African nations most threatened by Nigerian Islamist group Boko Haram must put aside mutual distrust and agree on a command structure and strategy for a fledgling regional force if they want to defeat the militants, a top UN official said.Mohamed Ibn Chambas, UN Special Representative for West Africa, said the international community could only help Nigeria and neighbouring Niger, Chad and Cameroon once they clearly laid out the assets they lacked to fight Boko Haram.A month before presidential elections in Nigeria, Boko Haram has seized swathes of new territory. It has killed hundreds of people in northern Nigeria, displaced several thousand more and seized the base of a regional military taskforce meant to fight it.The fall of Baga this month, where as many as 2 000 people are reported to have been killed, led to increased calls for international support to halt an insurgency that has spread from northern Nigeria to threaten parts of Niger, Cameroon and Chad."It is clear now that ... the countries should not be left to tackle it individually. That has been the approach so far and it is not winning the fight," Chambas told Reuters.He said it was up to the four states bordering Lake Chad to draw up better coordinated plans. He called for a clearer command structure and rules of engagement, amid resistance from some countries to see their troops deploy outside their borders under foreign command. Hot pursuit"They have not been able to agree the idea of joint operations and right of hot pursuit, which is very, very crucial in fighting a movement like Boko Haram which can engage in battle in one country and then run into another," he said.Chambas said defence and foreign ministers due to meet in Niger on Tuesday will have to agree on tough issues such as the ground rules and leadership of the regional military response."The challenge is to have the political will to agree and come to an understanding," he said.Foreign powers stepped up their support for Nigeria following the abduction of over 200 schoolgirls in the town of Chibok last year. However, Abuja has complained US authorities have not shared enough intelligence and been reluctant to sell weapons needed.France has offered to help structure the taskforce after promises by regional leaders last May to share intelligence and work together were undermined by the lack of trust.Chad on Wednesday offered to help Cameroon fight Boko Haram, days after it appealed for international help.Chambas said outside help would be needed for some specific capabilities but the priority was for regional armies to identify what they could put forward."It shouldn't be a problem of numbers of troops. They should be able to mobilize the troops in the first instance."