Beirut – New York-based watchdog, Human Rights Watch, has alleged that the Syrian government used chlorine gas on three towns in mid-April.
The watchdog cited interviews with witnesses and medical personnel, video of the attacks and photographs of the remnants of barrel bombs.
"Evidence strongly suggests that Syrian government helicopters dropped barrel bombs embedded with cylinders of chlorine gas on three towns in northern Syria in mid-April 2014," HRW said.
The group acknowledged it could not "independently confirm" that the chlorine gas cylinders caught on film were packed in the barrel bombs that were dropped from helicopters.
Opposition sources have claimed several government attacks using chlorine gas, but Syrian state television only acknowledged one such attack in Kafr Zita and blamed it on the jihadist Al-Nusra Front.
Doctors who treated victims said at least 11 people were killed in the attacks and "symptoms consistent with exposure to chlorine" were seen in nearly 500 people, said the report.
It documented attacks on the towns of Kafr Zita in central Hama on 11 and 18 April, Al-Temana in Idlib on 13 and 18 April and Telmans also in Idlib province on 21 April.
All three are areas under rebel control.