Air strikes struck a hospital in western Syria last week, killing two people, and there is evidence chemical weapons were used, Medecins Sans Frontieres (MSF) said on Friday.
A Syrian military source said this week allegations that government forces were using chemical weapons were "devoid of truth".
MSF said a bomb dropped from a helicopter hit the entrance of a hospital supported by the medical aid group in the rebel-held town of Latamneh in Hama province on Saturday.
"Information collected by the hospital medical staff suggests that chemical weapons were used," it said in a statement.
"Immediately after the impact, patients and staff reported suffering severe respiratory symptoms and burning of mucous membranes – symptoms consistent with an attack using chemicals."
The health authority of opposition-held parts of Hama blamed the attack on Syrian government warplanes. The Syrian government denies attacking hospitals and says it only targets militants.
The medical charity said the strike knocked the hospital out of service for three days, after which the emergency room reopened.
The attack killed an orthopedic surgeon, leaving nearly 120,000 people with only two such doctors, it added.
Jets have pounded rebel-held areas in rural Hama this week, as government forces fought to reverse the biggest assault by insurgent groups in months.
The Syrian Observatory for Human Rights, a UK-based war monitoring group, said the escalation in strikes injured at least 50 people in recent days.