There is strong evidence that the Syrian government used chemical weapons in helicopter-guided attacks that killed civilians in three rebel-held areas last month in the northwestern part of the country, Human Rights Watch reported Tuesday.

While both Syrian President Bashar Assad and anti-government rebels have accused each other since August of using chemical weapons, Human Rights Watch, a respected human rights group, said it determined that the April attacks were lodged by Assad’s regime in part because “the Syrian government is the only party to the conflict with helicopters and other aircraft.”

Based on interviews, and video and photo evidence, the human rights advocacy group said Syrian government helicopters dropped barrel bombs embedded with cylinders of chlorine gas in the mid-April attacks, and noted that the use of industrial chemicals as weapons is banned by the international treaty that Syria joined last October.

“Syria’s apparent use of chlorine gas as a weapon – not to mention targeting of civilians – is a plain violation of international law,” Nadim Houry, Human Rights Watch’s deputy director for the Middle East and North Africa, said in a statement. “This is one more reason for the UN Security Council to refer the situation in Syria to the International Criminal Court.”

Assad agreed to dismantle the government's chemical weapons by the middle of this year after it was determined forces loyal to him used sarin gas on rebel fighters in the months of March and April last year.  

The organization said it interviewed 10 witnesses, including five medical personnel, and reviewed video footage of the attacks. It also looked at photographs of the attacks’ aftermath, which “strongly suggest that government forces dropped barrel bombs containing embedded chlorine gas cylinders in attacks from April 11 to 21 on three towns in northwestern Syria.”

"Witnesses told Human Rights Watch they saw a helicopter dropping a barrel bomb or heard a helicopter immediately prior to an explosion, followed immediately by a peculiar odor.  The witnesses consistently described the clinical signs and symptoms of exposure to a choking agent … by victims,” the human rights group added.

The attacks in Keferzita, al-Teman’a and Telmans killed at least 11 people, according to doctors interviewed by Human Rights Watch.