US President Barack Obama is promising a vigorous investigation into reports Syrian forces fired chemical weapons and is renewing his warning that proof of their use will be a "game changer".
The White House said on Thursday the intelligence community had concluded with "varying degrees" of confidence that Syria's regime has used chemical weapons against rebels on a small scale.
Syria has denied firing such weapons and accuses anti-Assad rebels of using them.
Mr Obama said on Friday that the intelligence assessments were preliminary and Washington would work with its partners towards a definitive answer on the chemical weapons issue as soon as possible, AFP reports.
He said as horrific as it was that civilians face mortar fire and other attacks, the use of chemical weapons "crosses another line".
"That is going to be a game-changer ... we have to make assessments deliberately but I think all of us, not just the United States but around the world, have to recognise how we cannot stand by and permit the systematic use of weapons like chemical weapons on civilian populations.
"This is going to be something that we're going to be paying a lot of attention to, trying to confirm, mobilising the international community around those issues."