(CNN) -- Suicide attackers breached security at the African Union base in Somalia's capital by using vehicles with United Nations logos to carry out a deadly double car bombing, the organization told CNN.
"The vehicles had U.N. logos on them and they entered inside the headquarters and then exploded," said Gaffel Nkolokosa, spokesman for the African Union mission in Somalia, known as AMISOM. "We do not know if they were, in fact, U.N. vehicles."
U.N. Secretary-General Ban Ki-moon acknowledged that U.N.-marked vehicles were reportedly used in the attack, which he condemned "in the strongest possible terms."
He said the U.N. is investigating the attack.
Jean Ping, chairman of the African Union Commission, issued a statement strongly condemning the attack on the base in Mogadishu, saying it had killed "a number of peacekeepers."
It was unclear how many casualties were caused by the twin suicide car bombs, Nkolokosa said.
"We are assessing the situation to get the exact details," the AMISOM spokesman said.
The two cars packed with explosives rammed into a building housing peacekeepers from Burundi and Uganda, Ping said. Al-Shabaab, the Islamist militia with ties to al Qaeda, claimed responsibility for the midday attack, he said.
"This terrorist attack is further testimony of the continued threat posed by the elements opposed to peace and stability in Somalia and by their backers, within and outside Somalia, as well as of their readiness to do whatever they can to derail the current efforts and undermine the commendable progress that has been made over the past few months to bring about a lasting solution to the conflict in that country," Ping's statement said.
Al-Shabaab is waging a bloody battle in Somalia against the country's transitional government and is on the United States' list of terrorist organizations because of its ties to al Qaeda.
Somali civilians were also injured in the attack, Nkolokosa said.
The building that was targeted, in addition to housing African Union forces, operates as a field hospital for Ugandan forces and civilians in Mogadishu affected by the violence. It was a one-time military barrack that is part of Mogadishu's international airport, said a cameraman who cannot be named for security reasons.
There is a 3,400-strong African Union peacekeeping force in Somalia, made up of Burundian and Ugandan troops.
-- CNN's David McKenzie and Journalist Mohammed Amiin Adow contributed to this report.