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The Syrian government has been accused of using toxic chemicals in a number of barrel bomb attacks in Idlib governorate.

A report released by rights watchdog Human Rights Watch (HRW) on Monday said forces loyal to Syria's President Bashar al-Assad had used the lethal substances in up to six attacks, killing six people in one.

In three attacks photographs and video strongly indiciated a chemical attack, the group said, further citing witness accounts of a chlorine smell.

"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's deputy Middle East and North Africa director.

Doctors who treated victims of the bombings told HRW that symptoms included breathing trouble, burning eyes, a burning sensation in the throat, and other symptoms that were consistent with exposure to a choking agent.

Describing an attack on March 16 that killed six civilians, rescue worker Leith Fares said he heard a helicopter drop two barrels but heard no explosions.

"You know, we were at first actually happy," Fares said. "It is usually good news when there is no explosion."

Ten to 15 minutes after hearing the helicopter however, wounded began arriving at the field hospital in Sarmin, a local doctor told HRW.

On March 6, the UN Security Council adopted a resolution expressing concern over toxic chemicals used as weapons, and threatened measures including economic sanctions or force, if such substances were used again.

Idlib was taken from the Syrian government by a coalition of rebels, including the Nusra Front, in an offensive in late March.

The Syrian government was widely accused of using chemical weapons in an attack that killed hundreds of civilians in the Damascus suburb of Ghouta in August 2013.