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 US diplomat that The Associated Press obtained Friday.
The letter from US Ambassador Susan Rice to UN Secretary-General Ban Ki-moon also cited two other incidents of possible chemical weapons use by the regime of President Bashar al-Assad.
President Barack Obama authorised 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. Rice said Syria's use of these weapons "crosses clear red lines that have existed within the international community for decades."
But Ban said Friday he opposes the US decision to send weapons and that there can be no certainty of chemical weapons use in Syria without an on-the-ground investigation.
The UN 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," he said. This requires tracking chemical agents from the place they were used — through soil, air, blood and tissue samples — to the laboratories where they are tested to ensure against tampering.
Russia's UN Ambassador Vitaly Churkin echoed Ban.
Moscow's contacts with American experts "did not convince our experts that, in fact, the information that was presented was convincing enough to come to the definitive conclusion that government forces used chemical weapons," Churkin said.
Rice, who will become Obama's national security adviser in July, told reporters Friday that the US government is "very confident" in its assessment.
"We've taken two months to reach this through a very careful and deliberative process," Rice said.
The United Nations, the United States and Russia are all trying to get the Syrian government and opposition to a new international conference in Geneva to try to agree on a transitional government based on a plan adopted in that city nearly a year ago.
Rice said the United States views military aid to the rebels and efforts to promote a diplomatic solution "as two tracks of importance that we are pursuing in parallel."
The US, Russia and UN are scheduled to meet June 25 to continue discussions on a new Geneva conference. The opposition said recently it won't attend until it gets better arms.
In the letter to Ban, Rice said the US wanted to inform the UN of its "updated intelligence assessment, based on multiple, independent streams of information, that the Syrian government has used chemical weapons on a small scale against the opposition multiple times in the past year."
The US 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 neighborhood of Shaykh Maqsud, she said.
Rice 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 UN fact-finding mission include these incidents in its ongoing investigation and report, as appropriate, on its findings," Rice said.
Rice told reporters that the US has "no reliable, corroborated reporting to indicate that the opposition has acquired or used chemical weapons."
"We will leave other options on the table, in terms of additional US response should we deem that in our interest and necessary," she said. "But the Assad regime should know that the decision announced yesterday is a direct consequence of our conclusion that they have utilised chemical weapons against the opposition and their own people."
Rice said the US is not excluding other options, including a imposing a no-fly zone, "but at this stage no decision has been taken and ... that option has some downsides and limitations that we are very well aware of."