President Vladimir Putin's top foreign policy adviser said US officials had recently presented Russia with new information about Assad's alleged chemical weapons use.
"What was presented by the Americans does not look convincing to us," Yury Ushakov told reporters.
"The information that has been presented, the facts that have been presented do not look convincing to us," he stressed.
Ushakov said the chances of holding a Syrian peace conference that Russia and the United States proposed jointly on May 7 would be hurt by US President Barack Obama's decision to provide military support for the opposition.
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Asked if Obama's decision to start arming the rebels would prompt Russia to proceed with the delivery of the sophisticated S-300 missiles to the Damascus regime, Ushakov said: "We are not talking about this yet."
Putin has said that Russia has signed a contract for the S-300s' delivery without making any shipments yet.
The missiles are a worry for regional powers and the United States because they could complicate the imposition of a no-fly zone in the region.