U.S. State Department spokeswoman Victoria Nuland said Washington was seeing opposition forces in Syrian "make significant gains on the ground."
This week, officials at a Friends of Syria meeting in Morocco stated support of a Syrian opposition council formed last month in Qatar.
"It is clear that the regime's efforts to defeat the opposition militarily are clearly failing," added Nuland.
Russian Foreign Minister Mikhail Bogdanov said this week he was concerned rebel gains in Syria put the country's chemical weapons arsenal at risk.
"Sadly, a victory by the Syrian opposition cannot be ruled out," he said.
Alexander Lukashevich, Nuland's counterpart in the Kremlin, said Friday, however, that the foreign minister's comments were overstated.
"We never changed our position and will not do so in the future," he was quoted by state-run news agency RIA Novosti as saying.
Though at least 30,000 people have died in a civil war that's dragged on for close to two years, Moscow has said a political solution remains the best way forward.
U.S. Defense Secretary Leon Panetta told CBS News that he ordered 400 troops to Turkey to help man Patriot missiles slated for defense against the Syrian war. NATO forces maintain there are no plans for a no-fly zone over Syrian territory, however.