(CNN) -- A brazen, daylight suicide bombing on the African Union's peacekeeping mission in Somalia's capital has killed at least 21 people, mostly peacekeepers, the mission said Friday.
The attack came days after U.S. special forces killed Saleh ali Saleh Nabhan, pictured, in southern Somalia.
Suicide bombers, disguised in two U.N.-marked vehicles, Thursday rammed through the security gate of the mission's headquarters, which is attached to Mogadishu's airport.
They detonated their explosives just as Somalia's transitional government and mission representatives concluded a high-level meeting inside the compound, said the African Union's special representative for Somalia, Nicolas Bwakira. The attack killed four Somali civilians and 17 Ugandan and Burundian soldiers, including the mission's second in command, Burundian Maj. Gen. Juvenal Niyonguruza, he said.
The force commander, Ugandan Maj. Gen. Nathan Mugisha, was among the 40 wounded. He assumed command of the African Union Mission in Somalia -- known as AMISOM -- last month.
The bombs struck a building that also housed the offices of DynCorp International, a U.S. military contractor that supports the peacekeeping operation, according to Bwakira. It is unclear if any of DynCorp's employees were among those wounded in the strike, and a company representative could not be immediately reached for comment.
After the bombings, insurgents fired mortars at the African Union compound in an effort to maximize the number of casualties, Bwakira said.
Al-Shabaab, the Islamist militia with ties to al Qaeda, claimed responsibility for the attack, according to the African Union.
The strike happened days after U.S. special forces targeted and killed a senior al Qaeda operative in southern Somalia.
Analysts immediately hailed the death of Saleh Ali Saleh Nabhan on Monday as a major blow to al Qaeda's efforts to work with Al-Shabaab to try to gain a foothold in the lawless country.
Days later, however, Al-Shabaab struck back at the most prominent Western-linked target in Mogadishu -- the African Union peacekeeping mission.
The African Union has a 3,400-member peacekeeping force in Somalia, made up of troops from Burundi and Uganda. It operates under a U.N. mandate to support Somalia's transitional federal government.
The peacekeeping force is charged with protecting key government and strategic installations in Mogadishu, including the port, airport and presidential palace. It is the de facto military force of the weak, transitional Somali government.
Thursday's bombing was the deadliest attack on African Union peacekeepers in Mogadishu. It was also a show of force by Al-Shabaab to the West, particularly after the death of Nabhan, who was considered the main link between al Qaeda and Al-Shabaab.
Bwakira said he hoped it would "galvanize the international community ... to do more and do better" in helping restore order to Somalia.
"The international community needs to do more, we are not happy with what we have today," the African Union ambassador said. "We need more financial support, military support and the international community needs to deliver that support -- not just deliver lip service."
Suicide bombings have become more common as Al-Shabaab strengthens its ties with al Qaeda, but the bombers rarely are able to penetrate security as deeply as the attackers apparently did Thursday.
Western countries vowed not to let the attack deter their mission.
The United Nations, European Union, Arab League, United States and others issued a joint statement vowing "not be deterred by such criminal acts and (to) continue all our efforts to ensure the return of peace and stability in Somalia."
France helped evacuate 17 of those wounded in Thursday's bombing to a hospital in Nairobi, Kenya, its foreign ministry said Friday. Other wounded victims were taken to Nairobi by the Nairobi-based charity group AMREF.
France and the United States have been supporting Somalia's transitional government on security issues. The United States is supplying arms to Somali forces.
CNN's David McKenzie contributed to this report