Six people including three senior military officials were killed when a suicide bomber attacked a stadium in the Somali city of Galkayo on Friday ahead of the planned arrival of the country's prime minister, police said.

The Islamist militant group Al-Shabaab claimed responsibility for the attack, which took place as a crowd waited for Prime Minister Mohamed Hussein Roble to speak -- but he had not yet arrived.

-There was a heavy blast caused by a suicide bomber at the entrance of the stadium, six people died including three senior military officials- local police officer Mohamed Abdirahman told AFP.

Galkayo military commander Colonel Ahmed Dahir said the exact death toll was still unclear.

-The suicide bomber targeted senior military officials who stayed close to the entrance of the stadium- he said.

Witness Naima Ali was one of the people in the stadium waiting to see the premier when the blast hit.

-I saw smoke and dust and (there was a) heavy bang at the gate area, people panicked and it was hard to find a way out of the place for a few minutes- she said.

-I saw several badly injured and dead bodies of men, some them in military uniform, but I cannot say how many.-

The stadium is located in the south of Galkayo, the capital of the north-central Mudug region.

The city is divided between two self-proclaimed semi-autonomous states -- Puntland and Galmudug, which includes Mudug.

Al-Shabaab said in a statement that it targeted the prime minister in the attack, which it claimed had killed the commanders of two local units.

The Al-Qaeda-linked group, which is waging a deadly insurgency in Somalia and regularly targets military and government officials, has previously claimed responsibility for similar attacks in the region.

Those include a suicide attack that killed Mudug's governor and his security detail in Galkayo in May, as well as another that killed the governor of Puntland in similar circumstances in March.

Al-Shabaab was driven out of the capital Mogadishu in 2011 and lost most of its strongholds, but still controls vast swathes of the countryside.