UNICEF offers its heartfelt condolences to the families and colleagues of those killed and injured.
Attacks on humanitarian workers are unacceptable and threaten the delivery of urgently-needed aid to those in need.
The four aid workers, Mohammad Aimal, Shirley Case, Nicole Dial and Jacqueline Kirk, were killed when a group of armed men opened fire on their two vehicles as they travelled from Kabul to Logar just south of the capital. Another IRC staff member was critically wounded.
Their deaths bring to 23 the number of aid workers killed in Afghanistan so far this year, eight more than were killed in all of 2007.