The UN said on Friday that more than 321,000 Somalis had left the capital since February, about 100,000 people more than their previous estimate.
Nearly one third of the city's population has now fled the violence.
"This new huge increase in numbers comes mainly from new figures collected by an international non-governmental organisation in the regions of Galgaduud and Mudug, which were not accessible until recently, and also from an increase in the numbers of civilians fleeing the Somali capital," the UN refugee agency said.
More Ethiopian troops moved into Mogadishu on Friday to reinforce their colleagues after a suspected suicide bomber attacked their base the previous day.
"The situation is Somalia has gone from bad to worse after the intervention of Ethiopian troops"
Abed, Kumasi, Ghana
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Scores of people were injured in during Thursday's clashes, including a rocket attack on a crowded market.
"We have admitted 71 [people]... 41 of them are seriously injured and the other 30 had minor injuries," a doctor at the Madina hospital told Reuters said in reference to Thursday's casualties.
Witnesses also told the AFP news agency that eight people were killed when a mortar landed in a bus station in the south of the city on Thursday.
A Somali human-rights group said on Friday that at least 113 civilians had been killed during the last three days.
Ban Ki-moon, the UN secretary-general, says in a new report that a "coalition of the willing" may be needed to enforce peace in Somalia.
|Ban: A UN force might work if armed groups|
agree to outside monitoring [AP]
In a report to the Security Council made available on Friday, Ban called on the 15-nation body to consider in June whether a conventional UN peacekeeping force could succeed in the lawless East African country or something more was needed.
Ban said a UN force might work if fighting stopped in south-central Somalia and all or most armed groups and communities signed up to an agreement allowing for outside monitoring.
In that case, UN involvement "would primarily focus on technical assistance to the reconciliation efforts, as well as on reconstruction and development, supported by an appropriate United Nations peacekeeping presence".
But if the political process fails and violence gets worse, he said, "alternative options, including peace enforcement, should be considered".