Preliminary reports from the UN Office in Burundi shows that 170 people have been killed and between 6,000 and 7,000 civilians displaced since rebels began attacking the capital, Bujumbura, from 7 July.
In a statement issued from the UN headquarters in New York, Hua Jiang, the deputy spokeswoman for Secretary-General Koffi Annan, said the figures were given on Thursday in a report to the UN Security Council during a closed-door meeting.
The new director of the Africa I Division of the Department of Political Affairs, Haile Menkorios, briefed the council on the latest rebels attacks on Bujumbura.
Fighters loyal to Agathon Rwasa's faction of the Forces nationales de liberation have been shelling the city since Monday. The Burundian army was reported to have responded, and that it had killed 15 rebels.
After the meeting in New York, Council President Inocencio Arias, who is also the Spanish ambassador to the UN, said the council members strongly condemned the offensive on Bujumbura, and called for the unconditional and immediate end to the attacks.
On Thursday, the UN Office for the Coordination of Humanitarian Affairs (OCHA), reported that there were explosions of mortar rounds in densely populated sectors of the city, and that rebels and military officials had encouraged civilians to flee the conflict areas.
"The impact of continued fighting on civilians and social infrastructure is devastating," OCHA quoted Sunil Saigal, the UN Humanitarian Coordinator in Burundi, as saying.
Saigal said that no accurate figure existed for the total number of civilians wounded or dead.
On its part, the UN Security Council said the spread of fighting into Bujumbura punctuated a marked deterioration of security across the country.
"Now, 16 of Burundi's 17 provinces are subjected to sporadic fighting, looting and armed banditry," Jiang said.