Russia has offered to help the rebels with air support against the self-proclaimed Islamic State, or IS, if the United States provides information about rebel positions.
But opposition groups have been quick to reject the idea, saying Russia cannot be trusted.
For almost a month now, Russia has been carrying out air strikes in Syria, claiming it is bombing mainly IS targets.
But the Free Syrian Army says it has been a target of Russia's air strikes, so it has been quick to reject an offer of help from Russian foreign minister Sergei Lavrov.
"The Americans' refusal to coordinate their anti-terrorist campaign with us is a big mistake. We're seriously prepared for such a coordination, and we're ready to give air support to the patriotic opposition, including the so-called Free Syrian Army. But we need to get in contact with the people who have the authority to represent certain armed groups."
The Free Syrian Army's Issam al-Reis says Russia's offer has no credibility because the Russians, contrary to their claims, have not been targeting IS, also known as ISIS.
He has likened Russia's intervention in Syria on behalf of President Bashar al-Assad to its failed invasion of Afghanistan back in the 1970s.
"From the beginning, they are not serious and they are targeting non-ISIS areas, so how could we trust their involvement now? We don't need their help. The way how Assad brought the Russians was like the same way and the same manner that the Afghani government invited the Soviets in the 1970s. And they didn't leave (on) their own, and we consider that they will do the same for Syria."
As Syria's main ally, Russia says it wants Syria to prepare for parliamentary and presidential elections.
President Assad says he is willing to take part and is ready to run himself for president.
His position is unchanged after almost five years of conflict.
He insists a political solution to end the war is possible but says it will depend on eliminating what he calls "terrorist" groups.
The deputy of Russia's State Duma Federal Assembly, Sergei Gavrilov, met with President al-Assad and has reiterated that position.
"The first aim is the struggle with, and victory over, terrorism. And after that, the elections, parliamentary and presidential elections. But first of all, we must reach peace on Syrian land. And all countries at first -- Russia, China, Iran, Cuba, Venezuela -- must help the Syrian people to get peace and economic reconstruction."
Issam al-Reis from the Free Syrian Army says Russia's involvement is only helping prolong the conflict.
"Mr Vladimir Putin, he's assisting a regime that indiscriminately kills its own people. How could we trust the Russians to help? If the Russians are serious to come and find a solution to Syria, they should push Assad out of power, because he is the generator who produced all the extremists in Syria."
Louay Al-Safi, a member of the Syrian opposition, has told Al Jazeera television President Assad cannot be allowed to remain in power as part of any peace process.
"To begin with, he is now an alleged war criminal, because of the destruction he has brought to the country. I don't think a foreign power would have made as much destruction as he has done in Syria. So there is no possibility for Syrians to reconcile with the one who killed their kids and destroyed their cities."
United States Secretary of State John Kerry has met with Saudi Arabia's King Salman in Riyadh to discuss the Syrian conflict.
The US State Department says Mr Kerry and King Salman reaffirmed their commitment to a unified, pluralistic and stable Syria without President Assad as its leader.
They also promised to continue, and intensify, support to what they call the "moderate" Syrian opposition while the political track is being pursued.
