NEW YORK, April 14 (UPI) -- Evidence observed at several bombing sites in Syria indicate the government used chemical weapons over the span of two weeks in March, the Human Rights Watch said.
An investigation by the non-governmental organization evaluated six instances in which Syrian government helicopters dropped barrel bombs in the Idlib governorate between March 16 and 31.
Witness accounts, photos and videos strongly indicated chemicals were used in three of the attacks, while the other three instances require a followup, the group said in a news release Tuesday.
The Syrian Observatory for Human Rights said six civilians were killed in one attack March 16. The victims were a husband, his wife, their three children and their grandmother. Dozens suffered from suffocation when the bombs were dropped on the town of Sarmin in northwest Syria on Monday, according to the monitoring group.
A Syrian military sourced denied chemical weapons were used in the March 16 attack.
"We confirm that we would not use this type of weapon, and we don't need to use it," the source said at the time.
There's no conclusive evidence what chemical was used, but witnesses reported smelling chlorine and people near the impact sites had symptoms consistent with exposure to toxic chemicals. Doctors treating these patients said they had trouble breathing, burning eyes, burning sensation in the throat and coughing. Some patients had more serious symptoms, including pulmonary edema or fluid in the lungs.
"Syrian authorities appear once again to have shown complete disregard for human suffering by violating the global prohibition against chemical warfare," said Nadim Houry, HRW deputy Middle East and North Africa director. "The UN Security Council and countries that are members of the Chemical Weapons Convention need to respond strongly."
Gas canisters were found among the barrel bomb remnants at the impact sites of three of the attacks.
"Among the remnants, witnesses reported finding containers typically used for refrigerants in refrigerators and air-conditioners. Videos and photos from the aftermath of five attacks, including material shared by the Syrian Civil Defense, show containers of a size, shape, and design commonly used for refrigerants. These canisters are easy to refill with other gases and widely available in Syria," the release said.
In September, the Organization for the Prohibition of Chemical Weapons found "compelling information" indicating chlorine gas was used in an attack earlier in 2014 in northern Syria.
Andrew V. Pestano contributed to this report.