President Barack Obama has said the US has seen evidence of chemical weapons being used in Syria.
However, speaking after meeting Turkish PM Recep Tayyip Erdogan, he insisted it was important to get more specific details about alleged chemical attacks.
Earlier, residents of a north Syrian town told a BBC reporter how government forces had dropped poisonous gas canisters on them from helicopters.
The government has repeatedly denied claims it has used chemical agents.
It is unclear why the well-armed regime would deploy chemical weapons, which are illegal under international law.