* Kenyan forces advance to within 50 km of port city
* Kismayu was nerve centre of rebels' southern operations
* Capture of city may not deliver knock-out blow
By Abdi Sheikh
MOGADISHU, Sept 18 (Reuters) - Somali militant group al Shabaab has pulled its commanders out of the port city of Kismayu, leaving foot soldiers to defend its last bastion against advancing African troops, residents and Kenya's military said on Tuesday.
The al Qaeda-linked rebels have lost strongholds across southern and central Somalia in the past year in the face of advances by African Union forces, including Kenyan troops.
The Horn of Africa nation, a battleground in the U.S.-led war on militant Islam, is seen as a threat to regional stability.
While the capture of Kismayu - the hub of al Shabaab's southern operations - would likely weaken the rebels' military capacity and morale, it is unlikely to deliver the knock-out blow hoped for by Mogadishu and its regional allies.
Western diplomats expect the insurgents will retreat into the hinterlands and resort to guerrilla-style hit-and-run raids and urban bombings.
Kenyan forces have overrun several militant outposts to the north and southwest of Kismayu in the past two days, pushing to within 50 km (30 miles) of Somalia's second biggest city.
The abandonment of Kismayu by rebel commanders amounted to an acceptance of defeat, Kenya's military spokesman said. "Kismayu is poised for capture. We don't expect any resistance once we get to the city," Colonel Cyrus Oguna told Reuters.
Al Shabaab's spokesman for military operations, Sheikh Abdiasis Abu Musab, said the city would not be surrendered.
"We shall defend Kismayu. We have held off Kenyan troops for a year," he said, referring to Kenya's deployment of troops inside Somalia in October following a string of attacks on tourists and aid workers blamed on the rebels.
BRACED FOR CITY'S FALL
In Kismayu's narrow, whitewashed alleyways where minarets dominate the skyline, residents braced for the city's fall. Several hundred civilians on Tuesday fled on donkey carts, mini-buses and lorries, those staying put said.
"Al Shabaab have evacuated their families and supplies. Fighters are returning from the front lines (outside the city)," said one shopkeeper who gave his name as Hussein.
"We're not seeing many al Shabaab fighters, only a few men, nor do we see their luxury cars," said resident Ismail Sugow.
Kismayu residents said the militants' radio station, Radio Andalus, had stopped broadcasting on Monday, cutting off the rebel leadership from supporters in the city and beyond.
Al Shabaab said technical problems had forced the station off-air and denied reports fighters were fleeing.
The Kenyan military's Oguna said the noose was tightening on Kismayu as Kenyan troops closed in on the city from all sides. He declined to give a timeframe for a final assault.
Despite some draconian rules al Shabaab has imposed that include amputation of criminals' limbs and the banning of music and watching football, the rebels have retained pockets of support in areas under their control.
Local elder Suleiman Nur said there was widespread opposition to the Ras Kamboni militia group that is supporting the Kenyan and Somali government forces.
Ras Kamboni's leader Ahmed Madobe was a former Islamist commander and governor of Kismayu but was driven out of the city by al Shabaab in 2009.
"Al Shabaab and the residents drove out the Ras Kamboni militia. Some residents are ready to join al Shabaab in the fight or in the guerrilla attacks that follow," said Nur. (Additional reporting and writing by Richard Lough in Nairobi; Editing by Yara Bayoumy and Pravin Char)