Syrian regime used chemical weapons twice in Aleppo: U.S. letter

President Barack Obama arrives in the State Dining Room of the White House in Washington, Friday, June 14, 2013, where he hosted a Father's Day luncheon. (AP / Evan Vucci)

Syria's government used the nerve agent sarin on two occasions in the embattled city of Aleppo in March and April, according to a letter from a top U.S. diplomat that The Associated Press obtained Friday.

The letter from U.S. Ambassador Susan Rice to U.N. Secretary-General Ban Ki-moon also cited two other incidents of possible chemical weapons use by the regime of President Bashar Assad.

President Barack Obama authorized military aid to Syria's rebels for the first time Thursday after the White House announced it had firm evidence of chemical weapons use by Assad's regime.

But Ban said Friday he opposes the U.S. decision to send weapons and that there can be no certainty of chemical weapons use in Syria without an on-the-ground investigation. The U.N. chief reiterated his longstanding position that there is no military solution to Syria's two-year-old conflict, which has killed more than 93,000 people. He said increasing the flow of weapons to either side "would not be helpful."

"The validity of any information on the alleged use of chemical weapons cannot be ensured without convincing evidence of the chain-of-custody," Ban said.

Rice, who will become Obama's national security adviser in July, told reporters Friday that the U.S. government is "very confident" in its assessment.

"We've taken two months to reach this through a very careful and deliberative process," Rice said.

In the letter to Ban, Rice said the United States has determined that sarin was used in a March 19 attack on the Aleppo suburb of Khan al-Assal and in an April 13 attack on the neighbourhood of Shaykh Maqsud. She said unspecified chemicals, possibly including chemical warfare agents, were used May 14 in an attack on Qasr Abu Samrah and in a May 23 attack on Adra.

"The United States requests that the U.N. fact-finding mission include these incidents in its ongoing investigation and report, as appropriate, on its findings," Rice said.