Thomas Hammarberg told journalists that what had occurred in Tskhinvali during the attack on the breakaway republic by Georgian forces on August 8 should never be repeated.
Hammarberg is on a fact-finding mission to Georgia to look into claims of atrocities by both Georgian and Russian forces.
The UN High Commissioner for Refugees, Antonio Guterres, earlier called damage in Tskhinvali "overwhelming."
Figures for fatalities during the August 8 attack are not yet clear, with South Ossetian authorities saying some 1,500 people died. Russia has established 133 deaths, admitting however that makeshift burials had made determining an accurate death toll difficult.
The U.S.-based Human Rights Watch organization has accused both Russia and Georgia of "indiscriminate attacks" on civilians during the fighting.
Russia concluded its operation to "force Georgia to peace" on August 12.