Russia on Thursday rejected accusations that Moscow had bombed moderate rebel factions fighting President Bashar al-Assad instead of ISIS and had hit civilians.
"The rumours that the target of these airstrikes was not IS positions are unfounded," Russian Foreign Minister Sergei Lavrov told journalists in New York after meeting his US counterpart John Kerry.
Russia on Wednesday launched air strikes in war-torn Syria, its first overseas military engagement in nearly four decades after Russian parliament gave President Vladimir Putin permission to use force abroad.
The campaign was started "in response to the appeal of the Syrian leadership to help fight ISIL and other terrorist groups," Lavrov said, referring to the Islamic State group.
But France and the United States immediately expressed doubts that Moscow was targeting ISIS militants.
The head of Syria's main opposition group accused Moscow of aiming to support Assad's regime and killing 36 civilians in the central province of Homs.
Lavrov said the Russian military only went after "terrorist groups" and said that Moscow had requested that American officials back up their accusations with firm evidence.
"They expressed doubt, arguing that there is evidence, which we asked to show us, because we stand by our targets," Russia's top diplomat said, in the comments also released by the foreign ministry in Moscow.
"Talk began that civilians were hurt by air strikes. We have no such data," he added. "We carefully make sure that these target strikes are precise."
The defence ministry in Moscow said for its part that Russian fighter jets had carried out 20 sorties and struck "eight Islamic State targets" including a command post in the mountains.
Russia's Kommersant daily on Thursday quoted a military source as saying that the Russian deployment includes Su-24M and Su-34 bombers, Su-30 fighter jets and Mi-24 combat helicopters.