UN Assistant Secretary-General for Human Right Ivan Simonovic says the actual figure "is likely to be significantly higher."
The death toll had previously been estimated to be about 3,000.
Simonovic told the UN Human Rights Council in Geneva on today that the global body has seen a sharp rise in killings, particularly between mid-July and the end of August.
By the time of the Sept 5 cease-fire, an average of 42 people were being killed each day.