Russia says data provided by the United States on the Syrian government's alleged use of chemical weapons, including sarin nerve gas, is "unconvincing" and has warned that direct military aid to rebels would hurt peace efforts.

President Vladimir Putin's top foreign policy adviser said US officials had recently presented Russia with new information about president Bashar al-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.

Mr Ushakov says 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 to the armed wing of the opposition Syrian Military Council (SMC).

"Of course, if the Americans truly decide and in reality provide more large-scale assistance to rebels, assistance to the opposition, it won't make the preparation of the international conference easier," Mr Ushakov said.

The head of the Russian lower house of parliament's foreign affairs committee went even further than Mr Ushakov, bluntly accusing Washington of making up claims that Assad had used chemical weapons.

"Information about Assad's use of chemical weapons has been fabricated in the same place as the lies about [Saddam] Hussein's weapons of mass destruction" in Iraq, Alexei Pushkov said on Twitter.

"Why would Assad use sarin 'in small amounts' against the fighters? What is the sense?! In order to prompt outside intervention? It makes no sense," he wrote.

Key points The US says the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale multiple times

The US says the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale multiple times US intelligence estimates up to 150 people have died from detected chemical weapons attacks

US intelligence estimates up to 150 people have died from detected chemical weapons attacks Washington says the attacks cross its "red line" in the crisis

Washington says the attacks cross its "red line" in the crisis President Barack Obama has decided to further support rebel forces, including militarily

President Barack Obama has decided to further support rebel forces, including militarily United Nations says 93,000 people - including 6,500 children - have died in the Syrian conflict

US promises 'military support' for rebels

On Thursday US officials refused to rule out moving towards arming rebels or imposing a no-fly zone, but said Washington would provide backing to the SMC.

"The president has made a decision about providing more support to the opposition. That will involve providing direct support to the SMC. That includes military support," deputy national security adviser Ben Rhodes said, declining to elaborate.

Asked if Mr 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, Mr Ushakov said: "We are not talking about this yet."

Mr 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.

The United States dramatically toughened its stance on Syria on Thursday, accusing Assad of using chemical weapons against his people.

The US government announced it had reviewed intelligence reports and concluded Syrian regime forces had used banned arms, including sarin nerve gas, in attacks that killed up to 150 people.

Hawkish US lawmakers welcomed the Obama administration's change in position but Senator John McCain said the president needed to go further.

"We need heavy weaponry. We need the kind that can counter tanks, and we need surface-to-air missiles," he said.

Syria's main opposition National Coalition said in a statement issued by its US office that it "welcomes increased US assistance including direct military support".

"The support should be strategic and decisive in order to force an end to the violence and to achieve a political transition," it said.

Assad regime says US claims are 'lies'

Syria on Friday rejected Washington's claims that it had used chemical weapons in its fight against rebels.

"The White House published a statement full of lies about the use of chemical weapons in Syria, based on fabricated information, through which it is trying to hold the Syrian government responsible for such use," state news agency SANA quoted a foreign ministry official as saying.

Damascus also condemned the US administration's decision to provide military support to rebels.

"The American decision to arm armed terrorist groups demonstrates ... the direct involvement of the United States in the Syrian bloodbath," the official said.

"This raises serious questions about their good faith when it comes to finding a political solution in Syria."

AFP