The Kremlin declined to comment on Thursday on whether Russian troops are in combat in Syria, after sources in Lebanon told Reuters that Russian forces had begun participating in military operations there.
Bashar al-Assad's opponents in the West and among Gulf Arab states fear a considerable Russian military buildup is taking place in Syria to support the country's president. Moscow says all its military assistance to the Syrian army is in line with international law.
"The threat coming from Islamic State is evident... The only force capable of resisting it is the Syrian armed forces," Kremlin spokesman Dmitry Peskov said, reiterating Russia's position that its long-time ally Assad should be part of international efforts to combat the ultra-hardline Islamists.
Peskov said Russian President Vladimir Putin would talk about Syria and Islamic State during his speech to the UN General Assembly in New York later this month.
No meeting between Putin and US President Barack Obama in New York has yet been scheduled, he said.
On Wednesday, Russia said it had "military experts" on the ground. Two US officials told Reuters that Moscow had delivered a small number of forces to Syria in recent days. Defence analysts in Moscow say Russian intelligence must also be present on the ground.
Russia's respected Kommersant daily on Thursday said Moscow's advanced BTR-82A armored personnel carriers were among arms supplied to Damascus.
'Russia is piling pressure, playing a blackmail game'
Moscow has signalled several times in recent weeks it is interested in a meeting between Putin and Obama in New York. The White House says it is not aware of any meeting planned at the moment.
Moscow would play up any such meeting for its domestic audiences to cast Putin as a peacemaker and an indispensable partner for Washington in tackling international crises, even at a time of high tensions over Ukraine.
"Russia is piling pressure, playing a blackmail game," said defence analyst Pavel Felgenhauer, noting how a Moscow proposal for an anti-IS coalition involving Assad has lost impetus.
"They want to push others to consider it more seriously," said the expert, who is often critical of the Kremlin. "Or else fear that Moscow could use its forces there for other purposes."