Satellite images have confirmed that ethnic Georgian villages inside South Ossetia have been deliberately burned, US-based Human Rights Watch says.
It said analysis by UN experts showed the damage "was caused by intentional burning and not armed combat".
HRW researchers had "personally witnessed Ossetian militias looting and burning down ethnic Georgian villages".
The group said this was evidence of war crimes and urged Russia to prosecute those responsible.
The conflict in the region began on 7 August when Georgia tried to retake South Ossetia by force after a series of lower-level clashes.
Russia launched a counter-attack and the Georgian troops were ejected from both South Ossetia and Abkhazia.
The two breakaway regions have declared independence which only Russia has recognised.
On Friday, Georgia said it had decided to cut diplomatic ties with Russia.
The satellite images show active fires in five ethnic Georgian villages around the South Ossetian capital, Tskhinvali, on on 10, 12, 13, 17, 19 and 22 August - well after active hostilities ended in the area on 10 August, Human Rights Watch said.
"The images strongly indicate that the majority of the destruction in five of the villages - Tamarasheni, Kekhvi, Kvemo Achabeti, Zemo Achabeti, and Kurta - was caused by intentional burning.
"The high-resolution images of these villages show no impact craters from incoming shelling or rocket fire, or aerial bombardment."
The images were analysed by UN experts in Geneva, Human Rights Watch said.
"The expert analysis indicates clear patterns of destruction that are consistent with the evidence gathered by Human Rights Watch researchers working in the region," the human rights group said.
The researchers had spoken to several members of the Ossetian militias who "openly admitted that the houses were being burned by their associates, explaining that the objective was to ensure that ethnic Georgians would not have the houses to return to," it said.
"All of this adds up to compelling evidence of war crimes and grave human rights abuses," said Rachel Denber, deputy director of the Europe and Central Asia division of Human Rights Watch.
"This should persuade the Russian government it needs to prosecute those responsible for these crimes."
Researchers have suggested both sides may have violated the codes of war - using violence that was either disproportionate or indiscriminate, or both - claims that the International Criminal Court is currently investigating.
Russian prosecutors have announced they are opening criminal cases into the deaths of 133 civilians who they say were killed by Georgian forces.
And Georgia has filed a lawsuit against Russia at the International Court of Justice, based at The Hague, alleging the country had attempted to ethnically cleanse Georgians from the breakaway regions.