However, U.S intelligence services stressed that they were still not 100 percent sure.

“What we say is that our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin,” US National Security Council spokeswoman Caitlin Hayden said.

“This assessment is based in part on physiological samples. Our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts.”

President Obama has said the use of chemical weapons by Syria, or the transfer of stockpiles to terrorist groups, would be a “red line” that could trigger U.S. reaction.

“We go on to reaffirm that the President has set a clear red line as it relates to the United States that the use of chemical weapons or the transfer of chemical weapons to terrorist groups is a red line that is not acceptable to us, nor should it be to the international community. It’s precisely because we take this red line so seriously that we believe there is an obligation to fully investigate any and all evidence of chemical weapons use within Syria.”

Meanwhile, UN leader Ban Ki-moon renewed a call for Syria to let inspectors into the country.

Syria asked for a UN investigation but has since refused to let a UN team waiting in the region into the country.

Assad’s government only wants its claims that opposition rebels used chemical arms to be investigated. Ban has said the team should also look into opposition claims.

The Syrian civil war has persisted, with an estimated 70,000 dead.