Pro-government forces have launched a major dawn offensive in the Somali capital, Mogadishu, to seize territory from al-Shabab Islamist militants. Some 1,000 soldiers backed up by 20 tanks captured three al-Shabab bases, a senior security official said. A BBC correspondent in the city says it is the biggest joint government-African Union offensive since August 2011. Al-Shabab is under attack on several fronts, with troops from Kenya and Ethiopia also gaining ground recently. The AU said this was the first offensive it had taken part in outside Mogadishu. Troops from Djibouti have recently arrived in Mogadishu to bolster the 12,000-strong AU force, while the AU is asking the UN to approve a further 50% increase in troop numbers. Al-Shabab controls many southern and central areas of the country. map The al-Qaeda linked group made a \"tactical withdrawal\" from most of the capital last year but has continued to stage suicide attacks in the city. It confirmed that the pro-government forces had gained territory but vowed to hit back. BBC East Africa correspondent Will Ross says this operation appears to be a concerted effort to clear the Islamist militants right away from the Somali capital. But he says this conflict no longer has front lines and with al-Shabab carrying out suicide bomb attacks it will still be very difficult to make Mogadishu safe. On Thursday, six people were killed by a suicide bomb attack in a refugee camp. The victims included a security guard and a local aid worker, witnesses said. The bomb exploded just 20 minutes after a team of international journalists had left the Mogadishu camp. They had gone to the city to see the situation six months after the famine was first declared in parts of the country. About 300,000 people have since flooded into the city to seek food and shelter, as al-Shabab has banned most international aid agencies from areas they control. The UN says a quarter of a million Somalis were still suffering from the famine and the crisis was likely to continue for the next six or seven months.