Somali officials say a bomb blast in the main market in the Somali capital has left at least five security forces wounded.

The bomb was concealed inside a military pick-up truck in Mogadishu's main market Bakara, busy with shoppers preparing for the Muslim holy month of Ramadan on Tuesday, officials said.

Security forces opened fire in the air after the blast to disperse the converging crowds to minimise casualties.

Abdulaziz Abu Musab, a spokesman for al-Shabab, said the group claimed responsibility for the attack which he said had killed three officials and left three others wounded.

African Union (AU) forces expelled al-Shabab from Mogadishu in August 2011. The group lost control of the capital but retain the ability to stage lethal attacks in the city, and it still holds sway in large parts of the rural south.

In previous years, the month of Ramadan has seen a surge in al-Shabab attacks, with gunmen urged to carry out deeds by their preachers. Last month al-Qaeda-linked group attacked a UN base in Mogadishu, killing 22 people.

The government has said it needs more international training and other support to build up its security forces. For now, it still relies heavily on AU peacekeepers to protect vital facilities.