Western leaders rebuked Russian President Vladimir Putin for supporting Syria's Bashar al-Assad in his battle to crush an uprising, setting the stage for what could be a tense two-day G8 summit.
US President Barack Obama is expected to use his first face-to-face meeting with Putin in a year to try to convince the Kremlin chief to bring Assad to the negotiating table to end the two-year-old conflict.
British Prime Minister David Cameron, who is chairing the G8 summit, acknowledged there was "a big difference" between the positions of Russia and the West on Syria.
After Putin warned Obama of the dangers of arming Assad's foes who he claimed ate human body parts, Moscow said it would not permit no-fly zones to be imposed over Syria.
"I think we fundamentally will not allow this scenario," Foreign Ministry spokesman Alexander Lukashevich said in Moscow.
"All these manoeuvres about no-fly zones and humanitarian corridors are a direct consequence of a lack of respect for international law."
Western leaders criticised Russia, Assad's only big-power ally, for delivering arms to Assad while the rebels perished.
"How can we allow that Russia continues to deliver arms to the Bashar al-Assad regime when the opposition receives very few and is being massacred?" French President Francois Hollande said.
The United States, stung by recent victories for Assad's forces and their support from Hezbollah guerrillas in the civil war, said last week it would step up military aid to the rebels including automatic weapons, light mortars and rocket-propelled grenades.
The European Union has also dropped its arms embargo on Syria, allowing France and Britain to arm the rebels, though Cameron expressed concern about some of Assad's foes.
"Let's be clear – I am as worried as anybody else about elements of the Syrian opposition, who are extremists, who support terrorism and who are a great danger to our world," Cameron said.
Obama and Putin are due to meet at about 6:30 pm local time