Several armed groups have said they will stop fighting the government if sharia were implemented.
They include the Islamic party, an alliance of four groups that control parts of the capital and have influence in the port town of Kismayo.
But the al-Shabab group, which controls the main cities in the south, has said it does not recognise the legitimacy of the government.
Thousands of Somalis have been killed in battles between armed groups and pro-government forces in the past two years.
The chaotic Horn of Africa nation has been without an effective central government since clan leaders overthrew the government in 1991 and then turned their militias on each other.
In 2006, the Islamic Courts Union took over the capital Mogadishu and much of the south before Ethiopian troops supporting the shaky UN-backed government chased them from power.
The Ethiopians left Somalia at the beginning of this year as part of a peace deal and parliament elected Sheik Ahmed Sheik Sharif as president in January.
The former Islamic fighter has distanced himself from more hardline militias.