Russia is flying both military equipment and humanitarian aid to Syria, Russian foreign minister Sergei Lavrov said today.
The United States and some European countries have expressed growing concern over the cargo on flights from Russia to Syria.
Moscow has previously insisted in public that its flights to Syria are only humanitarian. Washington has put pressure on Greece and Bulgaria in recent days to deny Russia's requests to use their airspace for its Syria flights.
Lavrov said Moscow was supplying its Middle Eastern ally with equipment in accordance with existing contracts, but refused to comment on reports that it was deploying extra troops.
He once again confirmed that Russia has military personnel in Syria, but shed no light on what the US and NATO fear is a Russian military build-up at a Syrian airfield.
The Kremlin too declined to comment on 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.
Peskov said Russian President Vladimir Putin would talk about Syria and ISIS during his speech to the UN 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 today said Moscow's advanced BTR-82A armored personnel carriers were among arms supplied to Damascus.