On 23 April 2013, the New York Times reported that the British and French governments had sent a confidential letter to the United Nations Secretary General, claiming that there was evidence that the Syrian government had used chemical weapons in Aleppo, Homs, and perhaps Damascus. Israel also claimed that the Syrian government had used chemical weapons on 19 March near Aleppo and Damascus.[42] By 25 April the U.S. intelligence assessment was that the Assad government had likely used chemical weapons – specifically sarin gas.[43] However, the White House announced that "much more" work had to be done to verify the intelligence assessments.[44] On 24 April, Syria had refused an investigation team from the UN from entering Syria, though Jeffrey Feltman, UN under-secretary for political affairs, said that a refusal would not prevent an inquiry from being carried out.[45] On 23 March 2013, Syrian government unusually requested the UN send inspectors to investigate, an incident in town of Khan al-Assal, as it said rebels had used chlorine-filled rockets.[46] However, they later refused to extend UN investigation.[47]
4 June 2013, a U.N. report stated that there are "reasonable grounds" to believe that limited amounts of chemical weapons have been used in at least four attacks in the civil war, but more evidence is needed to determine the exact chemical agents used or who was responsible. Stating that it has not been possible "to determine the precise chemical agents used, their delivery systems or the perpetrator."[48][49] On 22 June the head of UN human rights investigation, Paulo Pinheiro, said the UN could not determine who used chemical weapons in Syria after the evidence had been delivered by the United States, Britain and France. However, the commission reported that there were "reasonable grounds to believe that chemical agents have been used as weapons".[50]
After clandestinely spending two months in Jobar, Damascus, several reporters for the French news media Le Monde personally witnessed the Syrian army's use of chemical weapons on civilians.[51][52]
On 13 June, the United States announced that there is definitive proof that the Assad government has used limited amounts of chemical weapons on multiple occasions on rebel forces, killing 100 to 150 people.[53] Sarin was the agent used with no proof that the rebels had access to such weapons. Deputy National Security Advisor Ben Rhodes did not confirm whether this proof showed that Syria had crossed the 'red line' established by President Obama by using chemical weapons. Mr Rhodes stated that: "The president has said that the use of chemical weapons would change his calculus, and it has." [54] Tests conducted by France confirmed the United States conclusions, according to the French government.[55] Russian Foreign Minister Sergei Lavrov said that "the accusations of Damascus using chemical weapons put forth by the USA are not backed by credible facts."[56] Russian foreign minister Sergey Larov stated that it makes no sense for Syrian government to use chemical weapons when the government already maintains a military advantage over the rebel fighters.[57]
http://en.wikipedia.org/wiki/2013_Ghouta_attacks