Lebanese soldiers fought Sunni Muslim gunmen in the southern city of Sidon on Monday in one of the deadliest outbreaks of violence fueled by sectarian divisions over the civil war in neighboring Syria.The army said 12 soldiers had been killed in clashes which broke out on Sunday after security forces detained a follower of the hardline Sunni Muslim cleric Sheikh Ahmed al-Assir. His supporters retaliated by opening fire on an army checkpoint.Security sources put the army death toll at 15, with 60 wounded. They said at least two gunmen were also killed in the clashes but soldiers were surrounding the mosque where Assir's supporters were based, making it difficult to verify details.The mosque showed signs of heavy damage from 24 hours of ferocious exchanges of rocket and gunfire.Sidon had been on edge since violence erupted last week between Sunni and Shi'ite Muslim fighters, at odds over the Syrian conflict which pits mainly Sunni rebels against President Bashar Assad, an Alawite from an offshoot of Shi'ite Islam.Tensions escalated further when the Lebanese Shi'ite militant group Hezbollah sent fighters into Syria to lead the recapture of a strategic border town by Assad's forces."The army has tried for months to keep Lebanon away from the problems of Syria, and it ignored repeated requests for it to clamp down on Sheikh Ahmed al-Assir's group," the military command said in a statement on Sunday."But what has happened today has gone beyond all expectations. The army was attacked in cold blood in an attempt to light the fuse in Sidon, just as was done in 1975," it said, referring to the year that Lebanon's own civil war began.Assir, whose supporters accuse the army of giving cover to Hezbollah gunmen, called for people across the country to join him and demanded that "honorable" soldiers defect.