NEW : U.N. official calls the attack a `` tragedy '' A truck full of explosives blew up in front a U.N. compound , local officials say . The blast occurred at about 1:30 p.m. on Friday outside the main gate to the compound, which houses offices and other facilities for international organizations like UNICEF and UNHCR. Afghan security forces kill four assailants , officials say .
NEW : U.N. official calls the attack a `` tragedy '' A truck full of explosives blew up in front a U.N. compound , local officials say . An hours-long gunbattle followed the blast . Afghan security forces kill four assailants , officials say .