WASHINGTON, Sept 12 (Reuters) - General Martin Dempsey, the chairman of the U.S. military's Joint Chiefs of Staff, spoke with Pastor Terry Jones by phone on Wednesday and asked him to withdraw his support for a film whose portrayal of the Prophet Mohammad has sparked violent protests - including one that ended with the death of America's envoy to Libya. "In the brief call, Gen. Dempsey expressed his concerns over the nature of the film, the tensions it will inflame and the violence it will cause," Dempsey's spokesman, Colonel Dave Lapan, told Reuters. "He asked Mr. Jones to consider withdrawing his support for the film." U.S. military officials are concerned that the film could inflame tensions in Afghanistan, where 74,000 U.S. troops are fighting. The Taliban earlier on Wednesday called on Afghans to prepare for a fight against Americans and urged insurgents to "take revenge" on U.S. soldiers over the film.