Russia on Thursday rejected accusations its air strikes in Syria targeted moderate rebels fighting President Bashar al-Assad instead of Islamic State militants, as it prepared for urgent military talks with the United States to head off possible clashes.

Senior US officials had expressed alarm after Russian warplanes began their first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979.

Foreign Minister Sergei Lavrov challenged the United States to provide proof that Russia's operation was not targeting "terrorists" and dismissed accusations by the Syrian opposition that it had caused civilian deaths.

US officials had accused Moscow of inflaming the four-year war in Syria and expressed doubt that Russia -- a close ally of Assad -- was attacking IS targets.

"The rumours that the target of these air strikes was not IS positions are unfounded," Lavrov told journalists in New York after meeting his US counterpart John Kerry.

"They (the American side) expressed doubt, arguing that there is evidence, which we asked them to show us, because we stand by our targets," he said, in the comments 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 targeted strikes are precise."

The head of Syria's main opposition group accused Moscow of killing 36 civilians in the central province of Homs, while US officials said it appeared the strikes were not in IS-controlled territory.

The defence ministry in Moscow said for its part that Russian fighter jets carried out 20 sorties and struck "eight Islamic State targets" including a command post in the mountains.

Washington complained that Moscow gave only an hour's notice of the strikes but the two sides seemed to put a brave face on the dispute and were preparing to hold military talks on the situation, perhaps as soon as Thursday.

"We agreed on the imperative of as soon as possible -- perhaps even as soon as tomorrow, but as soon as possible -- having a military to military de-confliction discussion," Kerry said Wednesday, appearing with Lavrov on the sidelines of the UN General Assembly.

- 'What is Free Syrian Army?' -

Russian senators on Wednesday voted behind closed doors to unanimously approve intervention in Syria after Kremlin chief of staff Sergei Ivanov said Assad needed Moscow's help in fighting terrorists.

The decision followed weeks of military buildup by Russia in Syrian government-controlled territory.

It remained unclear how much of the opposition fighting Assad's army -- including the moderate opposition supported by the West -- is considered by Moscow as a potential target.

Speaking to Russian news agencies late Wednesday, President Vladimir Putin's spokesman Dmitry Peskov said that Moscow was helping Syria to fight IS group and "other terrorist and extremist groups".

When asked whether Free Syrian Army, the Western-backed moderate rebels fighting both Assad and IS, is on Moscow's list of terrorist groups, Peskov said: "Does it exist, the Free Syria Army? Haven't most of them switched to IS group?"

"What is the Free Syrian Army, is it an official term? Are they official forces? It existed but whether it does now nobody knows for sure, it's a relative concept."

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, some of which flew in on September 18 as a transport escort.

Six bombers reached Syria circumventing Azerbaijan and flying over the Caspian Sea, Iran and Iraq.

- 'Ultimatums unrealistic' -

Russia and the West are in deep disagreement over Syria, with Moscow backing Assad while Western powers blame him for starting what has become a brutal war with more than 240,000 people dead and millions displaced.

France on Wednesday launched an inquiry into Assad for alleged "crimes against humanity" including kidnappings and torture.

Moscow however has never condemned Assad for civilian deaths and argues that he must be part of the political solution to the conflict.

"Life has shown that it is unrealistic to give ultimatums demanding that Assad leaves in a situation when the country is in such a crisis," Lavrov said.

The Russian media has been portraying the Assad regime as the only force heroically stopping the spread of the brutal Islamic State group, therefore requiring Russian help.

The Rossiya state television channel said Thursday that Syria's military is "launching an offensive in northern Homs where over 5,000 militants are hiding".

Russian pilots will help Syrian military pinpoint fortifications of "terrorists" and radicals, the report said, featuring combat footage from drones and images of Syrian soldiers shooting.

"Most of the moderate opposition rebels supported by Washington are switching to the side of the Islamists," it said.