(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. UNICEF, the U.N. aid agency focused on children, said it employed four of the dead. Four other workers were in serious condition, the agency said. The bomb went off as a vehicle was taking workers from their guest house to their office. The local police chief said the bomber was inside the vehicle among the U.N. staff. The bomber also died in the attack, according to authorities. Nick Kay, the special representative of the U.N. secretary-general for Somalia, condemned the attack on his Twitter account. "Shocked and appalled by loss of life," he said. Somalia's President, Hassan Sheikh Mohamud, said he was horrified by the attack. "This attack is not just targeted at the United Nations, but in attacking UNICEF, Al-Shabaab has also attacked Somali children. It is an attack against the future of our country and I condemn it in the strongest possible terms," he said. UNICEF called the slain workers "an integral part of UNICEF's work in Somalia, dedicated to improving the lives of others." Al-Shabaab, the al Qaeda-affiliated terrorist group based in Somalia, caused outrage this month by carrying out an attack on a Kenyan university. Nearly 150 people, most of them students, died in that attack. In the Sunday incident, three African Union troops were ambushed in the Lower Shabelle region. The African Union Commission for Somalia, or AMISOM, said Sunday that the troops died during a "spirited fight" with attackers. "This attack on AMISOM peacekeepers is part of the continuous effort to subvert Somalia. It is an attempt at disrupting the growth that is evident across all regions, by the enemies of the Somali people. The blood of our brothers will however not be shed in vain. AMISOM will remain committed to pacifying Somalia," said AMISOM Ambassador Maman S. Sidikou. CNN's Nana Karikari-apau and journalist Omar Nor contributed to this report.
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 .