A suicide bomb tore through a southern Afghan town killing six security guards on Wednesday while a British soldier and 20 militants died in fighting linked to a growing Islamist insurgency. The bomber, who was riding a motorbike, blew himself up alongside guards from a private Afghan company escorting a convoy of logistics for international troops, police said. "Six private Afghan guards were killed," said the southern Afghanistan border police commander, General Saifullah. The attack was near a bazaar in Spin Boldak, a town on a key border-crossing with Pakistan through which most of the supplies to foreign forces arrive.