United States needs to gather more evidence on use of chemical weapons in Syria before President Barack Obama can determine that a 'red line' has been crossed and necessary action needs to be taken, his spokesman said.

France's Foreign Minister Laurent Fabius had yesterday said that the deadly nerve agent sarin gas has been used several times during the conflict in Syria, citing tests carried out by a French laboratory.

However, learning from its past experience, United States wants to have more evidence on its use before it can contemplate the next set of actions.

"We need more information. Others have said, and I think it is worth noting, that the case for Saddam Hussein having weapons of mass destruction was stronger than the case initially presented for the use of chemical weapons in Syria. And I think the American people expect that we take that evidence and we build on it and we build on it until we know what we have and we can present it. And that's what the President is insisting we do," the White House Press Secretary Jay Carney said yesterday.

Obama had earlier warned that the use of chemical weapons or their transfer to a terrorist group by the government of President Bashar al-Assad would cross a "red line".
"The President made clear that he would consider the use of chemical weapons by Assad, by his regime, to be a game changer. I have said all along, as he (Obama) has and others, that we retain every option available to us to address this situation in Syria," Carney said.

Dismissing allegations that chemical weapons might have been used by Syrian opposition, the White House asserted that if at all used, it has been used by the Assad regime.

"We have made clear that we believe that if chemical weapons were in fact used in Syria, they were used by the Assad regime. We are highly sceptical of claims that the opposition used chemical weapons. The chemical weapons in Syria we believe have been and remain under the control of the Assad regime," Carney said.