The Kremlin declined to comment on Thursday on whether Russian troops were fighting 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 Syrian 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 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. Defense 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.