Western governments have expressed concern that Russian airstrikes in support of the beleaguered regime of Bashar Assad are targeting moderate Syrian opposition groups.
President Vladimir Putin said Russian warplanes struck positions belonging to Islamic State (IS) militants just hours after the parliament in Moscow unanimously voted to authorise military action.
But US defence secretary Ash Carter said the Russians appeared to have hit areas where IS is not active.
French defence minister Jean-Yves Le Drian said: "Curiously, they didn't hit Islamic State. I will let you draw a certain number of conclusions yourselves."
Prime Minister David Cameron said it would be a "retrograde step" if the Russians hit moderate opposition forces like the Western-backed Free Syrian Army.
His comments echoed US secretary of state John Kerry who said Washington would have "grave concerns" if the Russians hit areas where IS - also referred to as Isil - or al Qaida affiliated groups were not operating
"Strikes of that kind would question Russia's real intentions fighting Isil or protecting the Assad regime," he said.
The strikes follow a tense meeting between Mr Putin and US president Barack Obama in the margins of the United Nations General Assembly in New York on Monday.
The two leaders clashed over the survival of Assad, with Mr Putin arguing he was the best defence against IS and the militants while Mr Obama insisted that ultimately he would have to go as part of any political settlement.
Announcing the launch of airstrikes, Mr Putin - who has been building up Russia's military presence in Syria in recent weeks - said they had attacked and destroyed eight IS positions, and insisted Russia's role in the conflict would be limited .
"We certainly are not going to plunge head-on into this conflict," he said.
"First, we will be supporting the Syrian army purely in its legitimate fight with terrorist groups. Second, this will be air support without any participation in the ground operations."
However a senior US official said the strikes had taken place in the western part of Syria beyond the city of Homs - where there was no IS presence - and appeared to be aimed at other opposition groups.
Tensions between Washington and Moscow were further underlined when the state department disclosed that the only notification they received from the Russians came from an official in Baghdad who informed US embassy staff strikes would begin shortly.
The official requested that US aircraft - who have been mounting their own attacks on IS in Syria - should stay away from Syrian air space while the Russian airstrikes were under way.
Speaking during a visit to Jamaica, Mr Cameron said the UK Government would need to look "very carefully" at exactly what had happened on the ground in Syria.
"I have a clear view that if this is a part of international action against Isil, that appalling terrorist death cult outfit, then that is all to the good," he said.
"If, on the other hand, this is action against the Free Syrian Army in support of Assad the dictator, then obviously that is a retrograde step but let us see exactly what has happened."
Meanwhile, the head of the main Western-backed Syrian opposition group condemned "Russia's military aggression" in Syria.
Khaled Khoja, the leader of the Syrian National Coalition, said Russia was "using its military force to support the Assad regime's war against civilians. And it risks implicating itself in war crimes".