A prominent human rights group accused the Syrian government Wednesday of using toxic chemicals during a recent surge in attacks involving barrel bombs on rebel-held areas in northern Syria.

Human Rights Watch said chlorine gas was probably used in at least three bombing raids that targeted Idlib province in April and last month, after the area fell to a powerful new rebel coalition. That coalition and other insurgent groups have recently inflicted heavy losses on the regime of President Bashar al-Assad in the north and east of Syria.

Assad’s government has been accused by Western countries of using chemical weapons over the course of the four-year conflict, including an attack involving sarin gas in 2013 that killed hundreds of people in a suburb of the capital.

Regime opponents and ­activists allege that Assad’s forces have punished residents in rebel-controlled areas with barrages of the crude bombs, which are built from oil barrels or gas cylinders and can be filled with toxic chemicals such as chlorine gas. Barrel bombs have been dropped by regime helicopters and airplanes on residential areas, hospitals and markets, killing thousands of ­civilians, according to human rights groups.

Another group said two barrel bombings on Wednesday killed at least 24 people, including children, in Idlib and rebel-held areas of Aleppo province. The British-based Syrian Observatory for Human Rights said that it expected the death toll to climb from those attacks.

This file image made from video broadcast in 2013 on Syrian State Television purports to show a chemical weapons expert taking samples at a chemical weapons plant at an unknown location in Syria. (Uncredited/AP)

In its Wednesday report, Human Right Watch said evidence indicates that three attacks in April and May on towns in Idlib involved barrel bombs containing toxic chemicals. The group was unable to confirm the exact toxin used in the attacks, which it said killed two people and affected 127. But it cited chlorine as the likely culprit based on interviews with first responders and doctors, as well as an examination of photographs and videos.

The total number of attacks involving chlorine gas during that time is probably much higher, according to the report, which was released to coincide with the U.N. Security Council’s regular monthly meeting on chemical weapons in Syria. Citing evidence provided by doctors in Idlib, the group said 24 suspected chlorine gas attacks were carried out between May 16 and May 19, killing at least nine people and affecting over 500.

“While Security Council members deliberate over next steps at a snail’s pace, toxic chemicals are raining down on civilians in Syria,” Philippe Bolopion, Human Rights Watch’s U.N. and crisis advocacy director, said in a statement.

He said the Security Council should impose sanctions for the attacks.

In 2013, the Syrian government agreed to a deal brokered by the United States and Russia to eliminate its chemical weapons arsenal, forestalling potential U.S. airstrikes. The Syrian government, which denies using chemical weapons, agreed to join the Organization for the Prohibition of Chemical Weapons (OPCW) as part of the agreement.

Last month, reports emerged that OPCW inspectors found traces­ of sarin and VX nerve agent at a military research site in Syria, raising suspicion that the government had not eliminated its chemical weapons stockpiles.

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