(CNN) — A bomb blast rocked Somalia’s newly reopened national theater in the capital city Wednesday, killing two top sports officials.

Witnesses reported other casualties, but it was unclear how many.

Said Mohamed Mugambe, head of the Somali football federation, and Somali Olympic committee chief Adan Hagi Yabarow Wish were killed in the attack, a hospital official told CNN.

The bomb went off during a celebration for the first anniversary of Somali national television in Mogadishu, according to witnesses. Screaming people fled the theater and ambulance sirens filled the air.

The African Union Mission in Somalia (AMISOM) said the device had been detonated by a female suicide bomber who blended in with the crowd gathered for the occasion.

Several civilians were killed and injured but no Somali government ministers were hurt, the group said in a statement.

Sepp Blatter, president of football’s governing body FIFA, said he was shocked by the news of the sports officials’ deaths.

“I knew both men personally and can only say good things about their endless efforts to promote sport and football in their country. They will be sorely missed,” he said.

The Islamist group Al-Shabaab, which has been waging war against Somalia’s weak transitional government, claimed responsibility for the bombing via Twitter.

A post to the group’s Twitter page said the explosives had been planted before the gathering and denied that a woman had carried out the attack.

Brig. Gen. Audace Nduwumunsi, deputy force commander of AMISOM, condemned the bombing.

“We are united with the Somali government and stand firm together. This was a despicable crime against the Somali people, but it will not stop us achieving peace in Somalia,” he said.

“Yet again the terrorists’ methods show that they are enemies of peace and are foreign to Somali culture. By their attack they are trying to derail the hopes and dreams of the Somali people but they will fail.”

He urged anyone in Mogadishu with information about planned attacks to come forward.

The Islamist group announced in February that it was tightening its ties to al Qaeda and has long been considered a terrorist movement by the United States.

It has waged an insurgency against the feeble Transitional Federal Government since 2007 but has suffered recent setbacks in its heartland in southern Somalia.

AMISOM and government forces drove Al-Shabaab fighters from the center of Mogadishu last year, while Kenyan troops crossed into southern Somalia in October to hit back for a rash of kidnappings it blamed on the group.

But despite their efforts, Al-Shabaab has continued to launch terror attacks in the capital.

Last month, it claimed responsibility for an attack in which a suicide bomber detonated himself at a tea shop near Somalia’s presidential palace, killing two people.

A report released by the UK-based Royal United Services Institute think tank Wednesday highlights how al Qaeda is seeking to reinvigorate and expand its operations by strengthening ties with established Islamist groups such as Al-Shabaab.

Al-Shabaab is also increasingly working to recruit members from outside its borders, the report says, presenting a new challenge for Western nations.

“The group has sought to expand its recruitment network by tapping into the pool of foreign jihadists and radicalised individuals in the Middle East, Europe and the US,” author Valentina Soria writes.

However, this may be a sign of waning domestic support in the face of military operations and food shortages, she says, and there is no evidence so far that it will lead to attacks by Al-Shabaab on targets overseas.

— CNN’s Nima Elbagir contributed to this report.