At least 79 people have been killed and 149 injured after a bomb-laden truck was detonated in the outskirts of Somalia's capital, Mogadishu, during the city's Saturday morning rush hour.

So far no group has claimed responsibility for the attack but authorities are treating it as a terror attack.

In a preliminary statement to reporters, Omar Mohamud Mohamed, Mogadishu's mayor, said that most of those killed were civilians including two Turkish citizens.

The mayor said the exact number of the dead had not yet been established but police and witness reports placed the figure at more than 70. However, with dozens more injured, some critically, the number is expected to rise

In a statement released through his Twitter account, Ambassador Ahmed Awad, Somalia's minister of foreign affairs, confirmed that two Turkish citizens were among the casualties.

"My heartfelt prayers and sincere condolences to the families of the innocent citizens including two Turkish brothers who lost their lives in the heinous attack this morning. May the almighty God have mercy on them. They were students with ambition, and hardworking men and women," Ahmed said.

The attackers targeted a busy area at the city's Ex-Control checkpoint where a security barrier had caused a traffic build-up. This led to the large number of casualties, which included several university students travelling in a bus.

In the past, extremist group al-Shabaab has carried out similar attacks in Mogadishu but the group that claims links with the al-Qaida was pushed out of Mogadishu by the African Union Mission in Somalia (AMISOM).

Coming at a time when AMISOM troops are contemplating withdrawal from Somalia as they prepare Somali forces to take over responsibility for security, the latest attack at the heart of the Horn of Africa nation raises serious doubts about the ability of Somali forces to keep terrorist groups under check.

Security is a major concern in Somalia especially as the international community is trying to help the country hold a one-man, one-vote election as a replacement for previous clan-based power sharing arrangements. With the next elections expected in the 2020-2021 period, security forces have the daunting task of ensuring terror groups like al-Shabaab which had suffered several critical defeats do not spring back.

Otiato Opali in Nairobi contributed to this story.