A video purporting to show an American being beheaded in Iraq has been posted on a militant Islamic website. The hostage said he was from San Francisco and urged American troops to leave Iraq, it is reported.The al Qaeda-linked group led by the Jordanian Abu Musab al Zarqawi claimed responsibility for the killing. The man named himself as Benjamin Ford on the video. "My name is Benjamin Ford. I am from San Francisco, California," he says, dressed in a plain beige T-shirt as he sat on a chair. "We need to leave this country right now. If we don't, everyone is gonna be killed...