"Russia is not planning to sell [new weapons]," Foreign Minister Sergei Lavrov told reporters during a diplomatic visit to Poland on Friday.
Those previously-agreed to weapon sales could include deliveries of S-300 long-range, surface-to-air missile systems to Assad's forces.
Secretary of State John Kerry issued a stern warning to Moscow over the possible arms sales on Thursday.
The delivery of S-300 missile systems or any other similar weaponry would only worsen the ongoing civil war in Syria, which is entering its third year.
"I think we have made it crystal clear that we would prefer that Russia is not supplying assistance," Kerry said at a news conference in Rome.
Lavrov did not directly mention the S-300 as part of the weapons yet to be delivered to Syria, but said any future arms sales will be in compliance with international law and would be used for self-defense.
The potential deliveries of anti-aircraft missiles to Syria would be to allow the regime "the ability to protect itself from air strikes" by outside countries, Lavrov said.
The scenario of foreign nations carrying out airstrikes against government positions in Syria "something that is not an entirely fantastical scenario," the Russian official said.
Israeli warplanes already have launched strikes against targets inside the country in recent weeks.
Support on Capitol Hill is growing for U.S. military action in Syria, including deploying American fighter jets to take out Assad's anti-aircraft defenses.
Washington and Moscow have been at odds over Russia's military support for the Assad regime since the beginning of the war.
Congressional lawmakers launched numerous efforts to cut all ties between the Pentagon and Russian arms supplier Rosoboronexport, due to the company's weapons making their way into the hands of Syrian government troops.
Assad's forces have used heavy weapons and airpower, supplied by Russia and Iran, to batter rebel forces looking to topple the longtime leader.
The result is the war has now settled into a bloody stalemate that is threatening to spill over into neighboring countries, such as U.S allies Turkey and Jordan.
The suspected use of chemical weapons by the Assad regime against rebel forces crossing a "red line" set by President Obama, has further fueled congressional calls for U.S. action.
Lavrov's comments come as the U.S. and Russia had seemingly reached a breakthrough in ending the Syrian conflict.
Kerry and Lavrov agreed earlier this week to establish a bilateral peace council, in an attempt to bring the war to an end.
The goal of the council will be to set the parameters for a peaceful end to Assad's rule and hand over power to a transitional, coalition government.
Kerry and Lavrov agreed to the council during meetings held in Moscow between the two top diplomats on Tuesday.