The United States stands behind its assessment that the Syrian regime has used chemical weapons, vowing Friday there will be no repeat of the damaging mistake it made over Iraq.

And President Barack Obama will outline the proof to his Russian counterpart Vladimir Putin when they meet next week on the sidelines of a G8 summit.

Obama will lay out "all of the evidence and the reasons why we are so confident, and the assessment that was released by the White House on chemical weapons use," State Department spokeswoman Jen Psaki said.

The White House on Thursday for the first time said that the regime of President Bashar al-Assad had used chemical weapons, notably sarin gas, on multiple occasions against the rebels seeking to oust him.

"This assessment was made through multiple independent streams of information," Psaki told reporters, after the White House said it was going to toughen its aid to the rebels including direct military support.

Russia reacted warily though saying the US data was "unconvincing."

The Kremlin's top foreign policy adviser Yury Ushakov warned Washington against repeating the mistake it made when it invaded Iraq in 2003 after falsely accusing Saddam Hussein of stocking weapons of mass destruction.

Psaki said the US was confident there would be no repeat of the damaging scenes at the United Nations when then secretary of state Colin Powell avowed that Saddam had stockpiled chemical weapons.

Those charges were never substantiated and no chemical weapons were ever found in Iraq -- something which has haunted US foreign policy since.

"Let me just remind you, the secretary and the president, in a different role, of course, were there for the debates around Iraq," Psaki said, referring to Obama and Secretary of State John Kerry.

"You all have asked about why this is taking so long and... part of the reason is because they both felt, as did members of the national security team, it was so important to nail down the facts and feel confident in them before making a further assessment."

"That's why we did our due diligence to nail down the facts," Psaki added.

Earlier this month, France said it had "no doubt" that al-Assad's regime has used sarin gas in the Syrian conflict.

A UN report previously said there were "reasonable grounds" to believe both sides in Syria had used chemical weapons.