"Over the last 10 days we have seen a very serious deterioration and the ceasefire is about to collapse," Bassma Kodmani, a member of the High Negotiations Committee of the Syrian opposition, told Journal du Dimanche, saying "the use of barrel bombs has resumed".
"The US-Russian mission monitoring the ceasefire is powerless," she told the French newspaper.
A fragile ceasefire between Syrian President Bashar al-Assad's regime forces supported by Russian air strikes and the rebels brokered by the United States and Russia has largely held since February 27.
The truce does not include areas where the Islamic State group and Al-Qaeda's local affiliate Al-Nusra Front are present.
"A blow was dealt to the opposition, for sure," Kodmani admitted, adding Russia had "attacked the supply lines of the brigades of the moderate opposition on the ground until the cessation of hostilities intervened in February".
She said the withdrawal of Russian forces announced mid-March "indicates to those who support Bashar that this assistance will not be unlimited and unconditional."
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"The challenge is whether Russia will be able to dictate the terms of negotiations with Damascus," she said.
The Geneva talks aimed at ending the Syrian war are scheduled to resume on April 13.
More than 270,000 people have been killed and millions have fled their homes since the conflict erupted in March 2011.
The UN Security Council passed a resolution in December which paved the way for the Geneva talks and called for elections in Syria to be held 18 months after a transitional government is agreed.
The fate of President Assad is a major sticking point in the talks.
"We maintain that we must decide on a transitional authority with full powers, including those of President Assad, while the regime mentions a government of national unity with a few opponents and independents," Kodmani said.
"Nobody sees how to reconcile these two visions."
She said US President Barack Obama had "let the Russians take all the cards in the game, he has no political will, so the United States could afford to be more involved".