A powerful mine blast ripped through a bus packed with commuters and schoolchildren in northern Sri Lanka on Thursday, killing at least 62 people, the army said, blaming the Tamil Tiger rebels. The insurgents denied any role in the attack. The explosion - described as "huge" by military spokesman Brig. Prasad Samarasinghe _ was the worst single act of violence since the government and Tamil Tiger rebels signed a cease-fire in 2002, and renewed fears of a return to war. A senior rebel leader countered allegations that the Tigers were behind the blast, suggesting the attack could be "the work of forces seeking to create ethnic tension between the Sinhalese and the Tamil population."