At least 15 wedding guests were killed and several went missing when their bus was swept away by flood waters in Pakistan's Khyber Agency on Saturday.The incident occurred in the mountainous Bazar Zakhakhel area when the driver lost control of the bus due to flash floods in the region, Xinhua news agency reported.The exact number of persons on the bus is not known yet, but the search to find more bodies or possible injured people in the flood stream continues.At least 20 persons were killed in separate incidents of roof collapse and flash floods triggered by heavy rain in the country over the past one week.