Douala - Fighters from Nigeria's Boko Haram group clashed with soldiers in neighbouring Cameroon on Friday after raiding a village in a remote border region that aid groups say is turning into a war zone.The Islamist militant fighters attacked Kerawa in Cameroon's Far North Zone on Thursday afternoon, leaving at least 12 people dead, one official said, the same day as a similar raid nearby and a suicide bombing in neighbouring Chad."They [Boko Haram] slit the throats of between three and seven people yesterday and killed others," the official added.Cameroon's army clashed with the attackers on Friday, locals added, though there were no details on casualties."We are inflicting an extremely heavy death toll on Boko Haram's side," said Colonel Badjeck, the army's spokesman. "I forcefully insist that Kerawa is not in Boko Haram's hands."Boko Haram's Islamist guerrillas have made the swamplands of Lake Chad, where the borders of Chad, Cameroon, Niger and Nigeria meet, into a war zone, the United Nations refugee agency said last week.Boko Haram has waged a brutal six-year campaign for an Islamist state in northeastern Nigeria. After neighbouring countries joined an offensive against the group last year, the conflict has spilled across their borders, displacing tens of thousands of people.Boko Haram used Cameroon's impoverished Far North to stockpile supplies and recruits until the government cracked down on the group last year.Cameroon is also in an 8 700-strong regional force led by Nigeria against the militants, expected to be operational by the end of the year. The United States is sending military supplies and troops to the central African country to aid the fight.