The Somali capital, Mogadishu, has been hit by some of the fiercest fighting in the city in months, in what is seen as a major setback for the government.The clashes began shortly after dawn between Islamist al-Shabab militants and government forces, backed by African Union (AU) troops.The AU forces are reported to have used tanks and heavy artillery.Al-Shabab withdrew from Mogadishu in August and the government then declared that the city was under its control.The fighting occurred in the northern districts of Karan and Huriwa on Thursday.BBC East Africa Correspondent Will Ross says the fighting lasted for several hours as al-Shabab tried, but it seems failed, to seize ground.A spokesman for the Islamist insurgent group said four African Union soldiers had died, along with nine government troops. A spokesman for the government forces told the BBC they had killed six al-Shabab fighters.Neither claim has been independently verified.The fighting followed an attack by al-Shabab on Wednesday on a military training camp run by AU troops in Wadajir to the south of Mogadishu.That area had previously been considered relatively safe.Since the Islamists made their "tactical withdrawal" from Mogadishu, there have been several suicide attacks in the city.The most recent happened on Tuesday in the Hodan district - at least five people were killed.The latest clashes suggest many al-Shabab fighters never left the capital but melted into the community, our correspondent says.The African Union has about 9,000 troops in Mogadishu to prevent the internationally-backed government from being overthrown.Kenya also sent its soldiers into the south of Somalia in October, following a series of cross-border kidnappings which it blamed on al-Shabab.The Kenyan parliament on Wednesday agreed to integrate its troops in Somalia into the AU force.Somalia has endured more than two decades of war and for much of that time had no functioning government.Some parts of the country have been hit by a severe drought this year.