Heavily armed men backed by machine-gun-mounted pick-ups erected barricades and took up posts around the town of Jowhar, about 90km north of Mogadishu, on Friday as hundreds of residents fled on hearing the Islamists planned to assault the town, witnesses said.
As the rivals girded for battle, fierce unrelated fighting erupted in Baidoa, the seat of Somalia's largely powerless transitional government.
Aljazeera's correspondent in Mogadishu reported that at least 13 people were killed and 10 others wounded in the Baidoa clashes between guards of Abdullah Yusuf, the Somali president, and local militias belonging to al-Rahanwin tribe.
The gun battle broke out over differences over where to set up the barricades along the city's streets, Aljazeera said.
In Jowhar, the Alliance for the Restoration of Peace and Counter-Terrorism (ARPCT) commander Hassan Bhisow ordered the new deployments after reports that Islamist militiamen were gathering to the south and west.
Aljazeera's correspondent reported quoting Somali sources that fighters of ARPCT were not advancing on Mogadishu.
After consolidating their grip on Mogadishu on Thursday, militias of the Sharia Courts had halted their advance on the ARPCT stronghold of Jowhar in response to appeals by tribal elders not to attack the city.
Yusuf heads a largely powerless
transitional Somali government
The Islamist fighters were re-grouping in the towns of Balad and Walewein, according to residents there.
"We got word the Islamist militia were gathering in Balad and Walewein and are ready to attack us, so Bhisow sent enough fighters to villages around Jowhar to erect defences," one elder told AFP.
ARCPT fighters were stationed at the nearby Garsale trading post and along the key road leading to Walewein, about 70km southwest of Jowhar, witnesses said.