The Islamist rebels fell back on their last bastion on Wednesday, pouring hundreds of fighters into Kismayu and raising residents' fears of a violent showdown with advancing AU troops.
Locals said the al-Qaeda-linked insurgents drafted in militants from several outlying regions in southern Somalia. The reinforcements arrived in convoys of machinegun-mounted pickup trucks and set about building defences, witnesses said.
Somalia is a hotspot in US-led efforts to combat Islamist militancy and al-Shabab is the most powerful of Somali militias spawned by two decades of conflict in an unstable region.
The al-Shabab reinforcements arrived in Kismayu a day after locals said the militia's commanders had pulled out, leaving only a small number of fighters to defend the stronghold.
"Al-Shabab fighters have filled the bases and camps they abandoned in the past few days," said butcher Farah Roble.
"We're terrified. Al-Shabab looks determined to fight for Kismayu," he said.
Rumours swirled through Kismayu's winding alleyways that one of the militants' top three commanders was now in the city.
Resident Ismail Sugow said al-Shabab had drafted in fighters from nearby Bay, Bakool and Lower Shabelle regions. A second resident said the reinforcements came from "other parts of the country".
Earlier this week, Kenyan forces overran several militant outposts to the north and southwest of Kismayu, pushing to within 50km of Somalia's second biggest city.
Kenyan military spokesperson Colonel Cyrus Oguna said the final assault on Kismayu was inevitable.
"We have to be cautious, the way is littered with explosives. Nevertheless, it will happen. Kismayu will fall," he told Reuters by phone.
Defeat in Kismayu, a hub of al-Shabab operations throughout the group's five-year insurgency, would badly hurt the rebels' morale and weaken their capacity as a fighting force.
It might not deliver the knockout blow sought by Mogadishu and its regional allies. Western diplomats expect the insurgents to turn increasingly to guerrilla-style hit-and-run raids and urban bombings.
'Join the Jihad'
The AU force urged the militants to lay down their arms.
"Already a number of them have contacted us indicating their wish to cease fighting and we have assured them of their safety if they give themselves up to our forces," deputy force commander of the AU Mission in Somalia (Amisom) General Simon Karanja said in a statement.
Al-Shabab said Kismayu, about 500km south of the capital Mogadishu, was calm. Its radio station, Radio Andalus, was back on air after broadcasts stopped on Tuesday.
"All offices and businesses are open. We do not fear our enemies. They cannot just dash into Kismayu because we have strong defences," Sheikh Abdiasis Abu Musab, spokesperson for al-Shabab's military operations, told Reuters.
The UN refugee agency said about 4 000 civilians had fled Kismayu since Monday. Residents reported on Wednesday that al-Shabab were trying to stop locals from fleeing.
"Al-Shabab is taking to the mosques and ordering people to fight. They said Muslims have a duty to fight," said resident Sugow.
Al-Shabab could be heard test-firing their artillery on the city's outskirts, Sugow said.
Another resident, Hussein Nur, said the militants, who accuse the government of serving only Western interests and want to install a strict interpretation of Sharia (Islamic law), are whipping up fear among the local population.
"Al-Shabab said in the mosques: 'All Muslims should join the jihad as Kenyan troops will rape your wives and sisters and loot your property'," Nur said.
Kenya sent troops into Somalia, on its eastern border, in October to help crush the militants. Kenyans are expected to lead Amisom forces in an eventual assault on the port city. – Reuters