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 UN Secretary General Ban Ki-moon also cited two other incidents of possible chemical-weapons use by the regime of President Bashar Assad.
U.S. 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. Rice said Syria's use of these weapons "crosses clear red lines that have existed within the international community for decades."
'The validity of any information on the alleged use of chemical weapons cannot be ensured without convincing evidence...'
-- UN Secretary General Ban Ki-moon
'We've taken two months to reach this through a very careful and deliberative process'
-- U.S. Ambassador Susan Rice
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 UN chief reiterated his long-standing 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.
Russian 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 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.
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 U.S., 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 U.S. 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 U.S. has determined 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.
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.
The Syrian government initially asked for an investigation of the alleged chemical weapons attack on March 19 in Khan al-Assal, which it blamed on the rebels. But it has refused to allow a UN investigation team led by Swedish chemical weapons expert Ake Sellstrom into the country to conduct a broader investigation of other allegations raised by Britain and France.
-- The Associated Press