Somalia's al Qaeda-linked al Shabaab rebels tightened their grip on the south of the country on Wednesday when their main Islamist rivals said they had agreed to join them to fight the government.
Al Shabaab fighters had already seized much of the territory held by the rival group, Hizbul Islam, in fighting in recent weeks. Residents in the area south of the capital said Hizbul Islam's last strongholds had been under siege by al Shabaab for days when its leadership agreed to merge with the larger group.
"Hizbul Islam has completely joined al Shabaab," Hizbul Islam spokesman Mohamed Osman Arus said.
"This does not mean we were captured," he said. "We always had a common goal and now the objective is to increase our efforts to oust the so-called Somali government and its foreign allies."
The combined rebels control most of central and south Somalia as well as much of the capital, hemming Western-backed President Sheikh Sharif Ahmed's beleaguered government into just a few blocks in Mogadishu.
While Hizbul Islam and al Shabaab have often fought together against the government in Mogadishu, they were rivals in other parts of the country, including the southern port of Kismayu, which was eventually seized by al Shabaab.
Hizbul Islam was seen by many Somalis as less severe in its interpretation of Islam than al Shabaab. Its leader, Hasan Dahir Aweys, had criticised Al Shabaab for supporting Osama bin Laden.
The African Union, which maintains an 8,000-strong force of Ugandan and Burundian troops supporting the Somali government, said the merger of the rebel groups could lead to more violence.
"Islamists' mobilisation for war will only cause more death of Somalis," Barigye Ba-Hoku, the AU spokesman told Reuters. "We will continue to execute our mandate."
Over the past year, al Shabaab was perceived to have become more powerful than Hizbul Islam and took control of more territory. In recent months, Hizbul Islam had taken a harsher ideological stance, bringing it more in line with al Shabaab, which wants to impose a strict form of sharia law in Somalia.
More than 21,000 civilians have been killed in Somalia since the al Shabaab insurgency started in 2007.
Hizbul Islam was founded in February last year as an umbrella organisation of four groups led by Aweys, a cleric who has been an influential figure among Islamists.
Residents said Somali young women in camps for displaced people had already started fleeing for fear of al Shabaab's strict Islamic rules.
"I have sent my three daughters to Mogadishu. We are now under the control of al Shabaab," said Safia Abdi, a mother of six in Elasha, a camp on the outskirts of Mogadishu.
"Hizbul Islam was better, they were social. Al Shabaab is very harsh."