Story highlights The bombing is "an attack against the future of our country," Somalia's President says

UNICEF says the staff members' vehicle was hit by an explosion on its way to their office

Four wounded staff members are in serious condition, the agency says

(CNN) An improvised bomb exploded near a U.N. vehicle traveling near the northeastern Somali city of Garowe on Monday morning, killing six people -- including four aid workers for the international children's agency UNICEF.

Somalia's President called it "an attack against the future of our country."

The attack follows a separate incident Sunday in which three African Union troops died in an ambush in the Lower Shabelle region of Somalia

The terrorist group Al-Shabaab claimed responsibility for Monday's attack, which occurred about 8 a.m. in the administrative capital of the autonomous Puntland region, according to the United Nations Assistance Mission in Somalia.

Four of the victims were foreigners and two were Somalis, said Ahmed Abdullahi Samatar, the police chief of Puntland.

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