African forces backing Somali government troops said they took control of one of the Islamist rebellion's last strongholds, sparking intense clashes on the outskirts of Mogadishu.
Four African Union soldiers were wounded in Friday's offensive, led by Burundian troops in the force, on the sprawling Deynile neighbourhood northwest of central Mogadishu, which had been an al-Shabab safe haven for years.
The African Union Mission in Somalia (AMISOM) is a well-equipped force of more than 10,000 soldiers from Uganda, Burundi and Djibouti.
"Troops from the Somali army backed by troops from the African Union Mission in Somalia seized the district of Deynile and routed the al-Qaeda-affiliated al-Shabab terrorists," AMISOM spokesman Paddy Ankunda said in a statement on Friday.
But well after that statement, shooting could still be heard in the area in early evening, a humanitarian source in Mogadishu told AFP news agency.
Deynile commands access to the Afgoye corridor, an area some 30 kilometres northwest of Mogadishu which is controlled by the Shabab and is home to the world's largest concentration of internally displaced people.
"An important security operation was launched this morning around Deynile and the Afgoye corridor and we have cut off the road to Afgoye," Mohamed Osman Hanaf, a commander in the government forces, told journalists.
"Heavy clashes broke out when Shabab attacked our troops in Deynile," said Hanaf, who said the al-Qaeda-linked group had been defeated but did not provide any casualty toll.
No Shabab official was immediately available for comment.
"The situation is a bit confused and we have contradictory information on who controls what," the Mogadishu humanitarian source said early Friday evening.
"Our forces and AMISOM advanced and took control of most of Deynile, including the airstrip," Abdulahi Muhidin, another Somali officer, said.
"The Burundians and the Ugandans advanced with tanks towards Deynile and heavy clashes erupted," witness Ise Abdulahi said.
He said most civilians had deserted the neighbourhood before the clashes.
Over the past few weeks, thousands of civilians have streamed out of Deynile ahead of an expected offensive by AMISOM and Somali forces who have tightened their grip on Mogadishu since August.
"All movement of civilians in the Afgoye corridor has been blocked and AMISOM is carrying out military manoeuvres there," a local resident, Jumale Ahmed, said.
Some 410,000 people, around one third of all the displaced people in Somalia, were still living in the Afgoye corridor at the start of the year, fleeing war or drought, according to figures from the UN refugee agency.
Hospital under fire
Medical aid group Doctors without Borders (MSF) said the hospital in Deynile had come under fire Friday.
"The emergency ward and one part of an operating theatre in this hospital, where MSF has been working since 2006, were hit and major damage was caused," the group said in a statement.
All 19 patients and 48 staff are currently holed up inside a ward sheltering from the clashes, MSF said, calling on the warring parties to respect the hospital's neutrality.
The Shabab, who were recently recognised as an affiliate by al-Qaeda supremo Ayman al-Zawahiri, want to create an Islamic state in Somalia and have been battling the weak Western-backed transitional government for five years.
They once controlled up to 80 per cent of the vast Horn of Africa country but government forces backed by local militia, regional armies and AMISOM have been regaining ground in recent months.
Since abandoning fixed positions in Mogadishu in August, the Shabab have been chased out of most of their strongholds, with the notable exception of the southern port of Kismayo.