The BBC has been shown evidence apparently corroborating reports of a chemical attack in Syria last month.
A BBC correspondent who visited the northern town of Saraqeb was told by eyewitnesses that government helicopters had dropped at least two devices containing poisonous gas.
The government has vehemently denied claims it has used chemical agents.
The US had warned that such a development would be a "red line" for possible intervention.
President Barack Obama said he had seen evidence of chemical weapons being used in Syria - but it was important to get more specific information about what happened.