Georgia started the war last year with Russia amid provocation by Moscow, a commission investigating the cause of the conflict has found, a source close to the investigating team said on Wednesday.
"The responsibility is shared between the two parties concerned," the source said, on condition of anonymity.
Speaking just before the commission's findings were handed to the European Union, the source said they concluded that Georgia had sparked the war in August 2008 with its attack on the breakaway region of South Ossetia.
The investigating team, led by Swiss diplomat Heidi Tagliavina, insisted that Russia had escalated the conflict, he confirmed.
"The report contains quite clear and definitive answers to the questions asked about what happened, as well as an evaluation of the political context. No question is left unanswered," he said.
The source said both sides had been cooperative in responding to the commission's extensive list of questions, but that "there were nevertheless a lot of contradictions between the answers received from the two parties."
The commission does not want its findings to be used as a basis for any legal action, he said.
Both Georgia and Russia are likely to claim that the findings vindicate their actions during the five-day war in which at least 250 people were killed and some 118,000 others forced from their homes.
Officials and experts expect the European Union to refrain from pointing the finger.
Moscow's decision to send scores of tanks into South Ossetia has been widely condemned, as has its recognition of the independence of South Ossetia, and another breakaway Georgian region, Abkhazia.
But no one wants to anger Moscow as the European Union and NATO work hard to smooth over ties, and while Russia's help is needed to curb Iran's nuclear ambitions and fight insurgents in Afghanistan.
Yet the European Union cannot be seen to ignore violations of sovereignty and the territorial integrity of the ex-Soviet republic, whose chances of joining the world's biggest military alliance now lie in tatters.
The mission -- with a budget of 1.6 million euros (2.34 million dollars) -- was launched in December "to investigate the origins and the course of the conflict", including its conduct under international law and possible war crimes.