Yaounde - The Nigeria-based extremist group Boko Haram on Monday threatened neighboring countries Niger and Chad, warning the fighters were prepared to carry out suicide bombings in the countries sending troops to help fight the militants.The warning came as leaders from Niger and other countries in the region gathered in Cameroon's capital to finalise plans for a joint offensive against the militants who have stepped up their attacks in recent weeks.In a translation published by the SITE Intelligence Group, Boko Haram sharply criticized Niger for joining the effort and said the country was being dragged into a "swamp of darkness". Over the last 10 days, Boko Haram fighters have repeatedly struck the town of Diffa but not the capital."If you insist on continuing the aggression and the coalition with the government of Chad, then we give you glad tidings that the land of Niger is easier than the land of Nigeria and moving the war to the depth of your cities will be the first reaction toward any aggression that occurs after this statement," it said, according to SITE's transcript.A multinational force to fight Boko Haram is expected to be formally launched in coming weeks. Chad, Cameroon, Niger and Benin initially pledged to help Nigeria. On Monday, Burundi and Central African Republic also agreed to contribute troops to fight the militant group.The Islamic extremist group has fought a five-year insurgency against Nigeria's government, leaving 10 000 people dead last year alone. The violence has forced some 157 000 people to seek refuge in Niger, while 40,000 others have gone to Cameroon and 17 000 are in Chad, the UN said. Almost 1 million Nigerians are internally displaced, according to the country's own statistics.On Monday, leaders in Central Africa said that 10 member states had agreed to contribute most of the $100m needed to combat Boko Haram. They did not state how much had been raised nor how much is remaining despite calling for the creation of an emergency fund to bridge the difference.