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 has warned that such a development would be a "red line" for possible intervention.
However, President Barack Obama said the current intelligence on possible chemical weapon usage did not constitute sufficient proof.
In April, Saraqeb, a town south-west of Aleppo, came under artillery bombardment from government positions.
Doctors at the local hospital told the BBC they had admitted eight people suffering from breathing problems. Some were vomiting and others had constricted pupils, they said. One woman later died.
A number of videos passed to the BBC appear to support these claims, but it is impossible to independently verify them.
A doctor who treated the dead woman said her symptoms corresponded to organophosphate poisoning and that samples had been sent for testing.
Both the US and UK have spoken of growing evidence that the Syrian government has used chemical weapons.
Rebel fighters have also been accused of using them. They also have denied this.
The UN says estimates that the two-year-old conflict has left at least 80,000 people dead.