The two Islamist groups fighting the weak UN-backed Somali government, al-Shabab and Hizbul Islam, are to merge, according to reports.
The two had been allies but have fallen out over the past year, with Hizbul Islam losing ground.
Some see the merger as a takeover by al-Shabab, which has links to al-Qaeda.
A spokesman for the African Union force which is supporting the government and which has recently gained ground said the move would make no difference.
Maj Bahoku Barigye told the BBC that his troops had often been attacked by both these groups at the same time.
The Islamist groups together control much of south and central Somalia, while the government says it now runs more than half of the capital, Mogadishu.
The reported merger has not been confirmed by the leaders of the two groups, in particular Hizbul Islam leader Hassan Dahir Aweys.
BBC Somali service analyst Mohamed Mohamed says some officials in Hizbul Islam are not happy with the proposed merger, as they oppose al-Shabab's links to international jihadist groups, such as al-Qaeda.
Various armed groups have been battling for control of Somalia for two decades, leading hundreds of thousands of people to flee the country.