The US has said that the Syrian government of President Bashar al-Assad had probably used chemical weapons on a small scale in the country's civil war.
The White House insisted that President Barack Obama needed definitive proof before he would take action.
The disclosure created a difficult situation for Mr Obama, who has set the use of chemical weapons as a "red line" that President Assad must not cross.
US officials said the intelligence community believed with "varying degrees of confidence" that the chemical nerve agent sarin was used by Mr Assad's forces against rebel fighters.
The US is mindful of the lessons of the start of the Iraq war more than a decade ago.
The White House said the evaluation that Syria probably used chemical weapons was based in part on "physiological" samples.
Chemical weapons experts say sarin can be detected in human tissue, blood, urine and hair samples, or in nearby soil or even leaves.
But the chemical can dissipate within days or weeks, depending on heat, wind and other factors.
In Syria, US officials said the scale of the use of sarin appeared limited.
Nobody is "seeing any mass casualties" from the possible use of chemical weapons in Syria, one US intelligence official noted.
The US has resisted being dragged militarily into Syria's conflict and is providing only non-lethal aid to rebels trying to overthrow Mr Assad.
It is worried that weapons supplied to the rebels could end up in the hands of al-Qaeda-linked fighters.