Militants launched a major attack on the Iraqi city of Samarra on Thursday, killing six people and occupying neighbourhoods, in the latest display of their reach and the weakness of security forces. The militants, travelling in dozens of vehicles, some mounted with anti-aircraft guns, attacked a major checkpoint on the southeast side of Samarra, killing the security forces guarding it and burning their vehicles, witnesses said. They then took control of several areas of the city, **rth of Baghdad, according to witnesses, who reported seeing the bodies of both security forces and gunmen in the streets. The police ******r said security forces have withdrawn from other areas to defend a revered Shiite shrine in central Samarra, which was bombed in February 2006, sparking a brutal Sunni-Shiite sectarian conflict that killed tens of thousands.