Two militia men have died and two are injured in a skirmish in the northern outskirts of Somalia's capital, Mogadishu threatening a two-day truce.
Up to 200,000 inhabitants have been affected by the fighting
At least 140 people, mainly civilians, died in battles between warlords and Islamist militia in a week of clashes.
The violence was fuelled by a belief that the US was backing the alliance of warlords against the Islamists.
It is not known what caused the early morning shooting at a road checkpoint controlled by Islamist supporters.
The BBC's Mohammed Olad Hassan in Mogadishu says despite the incident, the ceasefire is holding in the city.
SOMALIA'S THREE RIVAL GROUPS
Gets arms from: Ethiopia, Italy (Source: UN report)
Gets arms from: Eritrea (Source: UN report)
Anti-terror alliance of warlords
Believed to get support from US
But the atmosphere is tense with many people believing both sides are seeking reinforcements.
On Monday, the warlord alliance set up checkpoints on the main road out of Mogadishu to the fertile Afgoye region, 30km west of the city.
They said this was done to stop the Islamists rearming and all vehicles are being checked thoroughly for weapons.
Since the truce, brokered by clan elders, came into effect on Sunday people have been recovering corpses from the streets.
Residents have been returning tentatively to the battle zone in the northern districts, such as CC, to assess the situation and decide whether to return.
Dozens of corrugated iron houses were destroyed in the fighting and most homes are riddled with bullet holes, our correspondent says.
The ceasefire came after President Abdullahi Yusuf called on ministers, many of whom are warlords, to stop leading their militias into battle.
In the town of Baidoa, 250km to the west, members of the transitional government held a session of parliament to discuss the situation.
The transitional government has not moved to Mogadishu because of security concerns, and controls only a small part of the country.
On Friday the United States called for a ceasefire.
Sheik Sharif Sheik Ahmed signed the deal on behalf of the Islamic fighters and Nuur Daqle signed for the secular Alliance for the Restoration of Peace and Counter-Terrorism.
However, it is not clear if the alliance is fully committed to the ceasefire.
Alliance spokesman Hussein Gutale Ragheh had been quoted as backing the ceasefire but has also reportedly said that the alliance had not signed it.
There are strong suspicions the US has been secretly funding the warlords, although Washington insists it has not violated the arms embargo on Somalia, says BBC Africa analyst David Bamford.
But a top US diplomat in Africa, Jendayi Frazer, acknowledged on Friday that the White House would work with those who can help "prevent Somalia becoming a safe haven for terrorists".
The fighting began when warlords, who had divided Mogadishu into fiefdoms, united to tackle a growing Islamist force.
This was the second round of serious fighting in Mogadishu this year. In March, clashes between the two sides killed at least 90 people.
Somalia has not had an effective national authority for 15 years since the ousting of President Siad Barre in 1991.