He said the two sides were using machine guns and rocket-propelled grenades.
"The situation is Somalia has gone from bad to worse after the intervention of Ethiopian troops"
Abed, Kumasi, Ghana
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Wednesday's fighting had killed at least five people, including a government soldier and three people struck by stray bullets.
Mogadishu's dominant clan, the Hawiye, had brokered a ceasefire more than a week ago to end the worst fighting here in 15 years.
Four days of bloodshed that started in late March killed hundreds of people - and possibly more than 1,000, according to a local human rights group.
The fighting started late last month when Ethiopian troops used tanks and attack helicopters in an offensive to crush Islamic courts fighters who were driven from power in December by Somali and Ethiopian soldiers backed by US special forces.
The US has accused the courts of having ties to al-Qaeda.
The UN refugee agency says some 124,000 people have fled Mogadishu since the beginning of February.
Geoff Wordley of the UN High Commissioner for Refugees, said 2,000 refugees from Somalia, most of them women, have crossed the border into Kenya in the past week to reach the Dadaab refugee camps.
"They have been arriving through informal routes,'' Wordley said, because the border between the two countries has been officially closed since January.
Dadaab is growing increasingly crowded, and a UNHCR official said on Thursday that cholera was being reported.