At least two people have been killed and seven wounded when a bomb ripped through a busy market in the Somali capital, a government official said.

It is not clear who carried out Monday's attack but in the past similar explosions in Mogadishu have been claimed by al-Shabab fighters.

The group had previously vowed to wage a series of attacks during Ramadan, the Muslim fasting month which started on Sunday.

On Sunday, al-Shabab gunmen shot dead three people and said the killings were just the start of the group's Ramadan campaign, the Reuters news agency reported.

"It was a bomb they planted in a small room in the market that the local government staff used as they collected tax," Ahmed Hassan, the district commissioner of Kaaraan district, told reporters.

The government and African Union peacekeepers have stepped up security to try to prevent assaults during Ramadan by al-Shabab, which has waged a seven-year campaign to impose its strict interpretation of Islamic law.

In the past year or so, al-Shabab has killed dozens of people in guerrilla-style assaults in Mogadishu, on UN offices, the presidential compound, parliament and the courts.

Source: Al Jazeera