A senior Russian general has said Syrian rebels now have anti-aircraft weapons, including US-made Stingers.
Gen Nikolai Makarov was quoted by the Interfax news service as saying the origin of the surface-to-air missiles should be "cleared up".
Russia is the biggest supplier of arms to its Syrian government ally.
Aerial bombardment of rebel-held towns continued on Wednesday, as the UN's Syria envoy prepared to brief the Security Council on ceasefire efforts.
Lakhdar Brahimi has been trying to arrange a ceasefire between rebels and government forces over the Muslim festival of Eid al-Adha, which begins on Friday.
"We have reliable information that Syrian militants have foreign portable anti-aircraft missile systems, including those made in the USA... it should be cleared up who delivered them," Gen Makarov told journalists in Russia.
There have been earlier unconfirmed reports of the Syrian opposition having shoulder-mounted missiles, but the West has been reluctant to openly arm the rebels.
In August, Syrian rebels said they had shot down a fighter jet near the border with Iraq.
Syrian warplanes have stepped up their bombardments of rebel-dominated areas in recent months, particularly in the north of the country. Deadly air raids are now daily events in towns around the city of Aleppo.
Recent footage has emerged of Syrian opposition fighters using old Soviet SA-7 heat-seeking missiles, which can destroy a plane flying at up to 14,000ft.
US-made Stinger missiles are shoulder-mounted anti-aircraft weapons designed to target low-flying planes and helicopters.
A US decision to supply them to the mujahideen in Afghanistan in the 1980s to fight the Russians proved to be a turning point in the war.
The UN says that more than 18,000 people have died so far in the uprising against Bashar al-Assad's government, which began in March 2011, but activists and opposition groups put the figure closer to 30,000.