Human Rights Watch latest to claim Syria's Assad used chemical weapons in April

May 12, 2014: A vehicle drives past campaign posters for the June 3 presidential election in Damascus, Syria. (AP Photo)

BEIRUT – A leading international human rights group said on Tuesday it has strong evidence that Syria's army used chlorine gas on rebel-held neighborhoods last month, dropping the canisters in crude bombs on residential areas.

The statement by the New-York based Human Rights Watch adds to growing concerns that chemical weapons are still being used in Syria -- months after a chemical attack killed hundreds of civilians last August.

Human Rights Watch said forces loyal to President Bashar Assad likely used chlorine gas on three towns in northern Syria in mid-April, according to interviews with 10 witnesses, video footage and photographs.

"Evidence strongly suggests that Syrian government helicopters dropped barrel bombs embedded with cylinders of chlorine gas on three towns," said the group.

"These attacks used an industrial chemical as a weapon, an act banned by the international treaty prohibiting chemical weapons that Syria joined in October 2013," it added.

In late April, the U.N.'s chemical watchdog said it will investigate the chlorine claims. The Organization for the Prohibition of Chemical Weapons has not commented further on the issue.

In one incident, the Syrian government blamed the Al Qaeda group, the Nusra Front, for using chlorine gas on civilians in the rebel-held town of Kafrzeita. It has not commented on other attacks.

An extensive Associated Press investigation in late April found consistent claims that chlorine gas had been used in Kafrzeita.

Human Rights Watch said testimony from eye-witnesses indicated that chlorine canisters were embedded into crude explosive-laden barrels, which military helicopters dropped at the time on rebel-held areas.

In Syria, only the pro-government forces have military aircraft, not opposition fighters. And though chlorine gas canisters are widely available, Human Rights Watch said their use as a weapon is prohibited under international law.

The use of chlorine gas in bombs is not very effective as a weapon to kill people. However, HRW said it appeared the Syrian military was using the chlorine to terrorize residents into believing they had been gassed, even if many of the victims were not killed.

The Syrian government narrowly avoided Western-backed airstrikes after the August attack in areas of rural Damascus that killed hundreds of civilians.

Instead, the U.N.'s security council ordered Syria to dismantle its chemical weapons infrastructure and destroy its arsenal, by June.