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 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 that 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 neighborhood of Shaykh Maqsud. She said that 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.
Syria initially asked for an investigation of an alleged chemical weapons attack on March 19 in Khan al-Assal, which it blamed on the rebels.
But it has refused to allow a U.N. team, led by Swedish chemical weapons expert Ake Sellstrom, into the country to conduct a broader investigation of that incident and other allegations raised by Britain and France.