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The Kremlin declined to comment Thursday on whether Russian troops were fighting in Syria.

Bashar 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 ISIS 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.

Related: NATO Voices Concern About Russian Presence in Syria

Peskov said Russian President Vladimir Putin would talk about Syria and ISIS during his speech to the U.N. General Assembly in New York later this month. No meeting between Putin and President Barack Obama in New York has yet been scheduled, he added.

On Wednesday, Russia said it had "military experts" 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.