Al-Shabab has been fighting the government for more then three years [File: AFP]

At least 21 people have been killed in the Somali capital of Mogadishu after fighters from an armed group clashed with African Union troops.

Scores more people were injured in the battles, which included artillery shelling and gunfire, on Wednesday, Ali Muse, an ambulance service chief, said.

Witnesses said that African Union (AU) troops shelled the area of Bakara market, the city's largest market, following an attack on one of their convoys.

"The medical teams got an emergency alert and they rushed to Bakara market where they have collected around 16 dead bodies of civilians who were killed in the shelling," Muse said.

Convoy attack

At least five other civilians were killed in crossfire after a roadside bomb and an ambush hit an AU peacekeepers convoy, witnesses said.

AU officials said that the attack on their convoy had occurred but did not comment on any civilian casualties.

Al-Shabab, a key group fighting the government, said a day earlier that they would continue attacks against anyone who opposed their rule in the country, which extends to much of Mogadishu and large parts of the south and centre of the country.

Ali Muhamud Hussein, al-Shabab's self proclaimed mayor of Mogadishu, said: "We are prepared to fight every single infidel wherever he or she is, including as far as Moscow.

"We need to terrorise all unbelievers anywhere."

Al-Sahbab has fought Somalia's UN-backed transitional government for more than three years.

The current government of Sheikh Sharif Ahmed, the president, controls only parts of the capital.

There has been no effective central government since 1991 in Somalia which has undergone periods of war for much of that time.