Russian media are reporting that an arms industry source says Russia has not yet delivered anti-aircraft missiles to Syria and may not do so for several months.
The source told the Russian news agency Interfax any delivery of the advanced S-300 air defense system missiles would be made no earlier than September.
On Thursday, Syrian President Bashar al-Assad hinted he had received new weapons from Russia and threatened retaliation against Israel if it carries out future airstrikes on his country.
Russia had vowed to proceed with the transfer of anti-aircraft missiles, saying it will help deter foreign involvement in Syria's civil war.
In an interview with Lebanon's Al-Manar television, Mr. Assad was vague on whether the new weapons include the S-300 air defense system.
"All we have agreed on with Russia will be implemented and some of it has been implemented recently. And we and the Russians continue to implement these contracts."
Israel has threatened to use force to stop the the transfer. That country's main civilian airport would be within range of the S-300. Earlier this month, Israel attacked a suspected weapons supply in Damascus it says was headed for the Lebanese militant group Hezbollah.
Meanwhile, Syrian troops and their Hezbollah allies have come closer to capturing the town of Qusair, which controls supply routes vital to the army and rebels, and provides access between Damascus and the Mediterranean.
Local media reported troops have cut off the supply routes of rebel forces in the Arjun district in northern Qusair. Pro-Assad forces have been surrounding Qusair in recent days.
Also, a U.S. citizen was killed in Syria while fighting for opposition forces. Syrian state media reported the woman, Nicole Mansfield, died in fighting in the town of Idlib.
The U.S. Federal Bureau of Investigation confirmed the death to Mansfield's family.