A UN report on Syria said Tuesday there are "reasonable grounds" to believe that limited quantities of toxic chemicals have been used as weapons in at least four attacks in Syria's civil war, but said more evidence is needed to determine the precise chemical agents used or who used them.
The UN Commission of Inquiry said conclusive findings can be reached only after testing samples taken directly from victims or the site of the alleged attacks. It called on Damascus to allow a team of experts into the country.
U.S. President Barack Obama has said repeatedly that Syria's use of chemical weapons, or the transfer of its stockpiles to a militant group, would cross a "red line." Tuesday's report appeared to strengthen U.S. officials' assertions that more definitive proof was needed.