At least 18 persons were killed when a suicide car bombing struck a crowded area in Afghanistan's Khost city on Saturday, an official said.Ten security guards and eight civilians were among the dead in the attack that targeted local off-duty security guards, known as Campaign Forces, Xinhua news agency quoted the security official as saying.The blast took place near a taxi station where the guards arrived to go to Camp Chapman, a military base run by the US and NATO forces, he said.Police chief General Faizullah Ghairat said the police have launched an investigation into the incident that also left about six persons injured in the attack in Khost, capital of Khost province bordering Pakistan.No group has so far claimed responsibility for the attack.The attack came as Afghans mark the first day of Ramadan, the Muslim holy month, during which the faithful fast from dawn to dusk.