UPDATE 1-Big explosion heard in Somali capital Mogadishu - Reuters witness

A big explosion was heard in the heart of Somalia's capital Mogadishu on Saturday, with a plume of smoke seen above the scene of blast, a Reuters witness reported.

It was not clear who was responsible for the blast and what exactly had been detonated although Islamist group al Shabaab frequently carries out bombings in the Horn of Africa country. "The first blast took place at the Ministry of Labour ... there are also other ministry buildings near the scene. It was a suicide car bomb followed by gunfire," said police officer Major Mohamed Hussein.

He added that a second blast had followed the initial one. Al Shabaab is fighting to topple Somalia's western backed central government and establish its own rule based on its strict interpretation of Islamic sharia law.

The group was ejected from Mogadishu in 2011 and has since been driven from most of its other strongholds across the country. But it remains a formidable threat, with its fighters frequently carrying out bombings in Somalia and neighbouring Kenya. Troops from Kenya form part of the African Union- mandated peacekeeping force AMISOM that helps defend Somalia's central government.

(Reporting by Abdi Sheikh; writing by Elias Biryabarema; Editing by Alexander Smith and Louise Heavens)