If the rebels, known as the Free Syrian Army, are supplied with American arms and given air support by U.S. warplanes, "the carnage will go on for many years," Russian Foreign Minister Sergey Lavrov told reporters.
Lavrov plans to meet with a moderate faction of the Syrian opposition in Moscow within the next two weeks, according to reports in The Associated Press.
The United States has agreed to ship non-lethal aid to elements of the Syrian resistance.
Run exclusively by the State Department, those supply shipments will reportedly include communications equipment and military-grade night-vision equipment.
Department of Defense spokesman George Little declined to comment on the shipments during Tuesday's briefing with reporters.
"This is a State Department led effort," Little said, stressing that any materiel handed over to the Syrians would be strictly non-military.
The shipments are part of a larger peace pact drafted by U.N. envoy to Syria Kofi Annan and the Friends of Syria group, an 80-country coalition including the United States.
Congress remains divided on whether to arm anti-Assad forces in Syria.
Advocates of military support on Capitol Hill claim armed support is the only way to curb Assad's brutal crackdown on rebel forces.
But other lawmakers are wary that such intervention might draw the United States deeper into another war in the Middle East, just as American troops are preparing to come home from Afghanistan.
Questions also remain on whether the United States and its allies can guarantee any weapons handed over to the Free Syrian Army won't end up in the hands of terror groups like al Qaeda.
Moscow has rejected the Annan deal because it puts a firm deadline on when Assad should step down from power.
Russia, along with Iran, has backed the Assad government since the uprising began last February as part of the Arab Spring movement in the Middle East.
Russian defense firms have been supplying arms to government forces, while the Kremlin has sent warships to the Syrian port of Tartus over the past few weeks.
The country reportedly sent elite units marines and special-operations forces to Syria in March to conduct "anti-terrorism" missions in the country.