BEIRUT — An international human rights group said Tuesday it has strong evidence that the Syrian army used chlorine gas on three rebel-held towns last month.
The statement by the New-York based Human Rights Watch adds to concerns that chemical weapons are still being used in Syria, months after a chemical attack killed hundreds of civilians last August.
The Organization for the Prohibition of Chemical Weapons, which monitors implementation of the Chemical Weapons Convention, said in April that it would investigate the chlorine claims but hasn't commented further.
Human Rights Watch said in a report released Tuesday that forces loyal to President Bashar Assad likely used chlorine gas packed into crude bombs in attacks in mid-April on three towns near a military base in northern Syria.
Those attacks killed at least 11 people, and wounded as many as 500. The HRW said its report was based on interviews with 10 witnesses, video footage and photographs.
Chlorine gas in bombs is not very lethal, but HRW said it appeared to have been used to terrorize residents into believing they had been gassed, to cause widespread panic.
"Evidence strongly suggests that Syrian government helicopters dropped barrel bombs embedded with cylinders of chlorine gas on three towns," said the group.
The Syrian government did not immediately comment on the HRW claims. It has accused rebels of using chlorine gas in the past.
Human Rights Watch said testimony from eye-witnesses, including medics, showed the wounded suffered from exposure to chlorine gas, and there were canister fragments that showed the labels of factory where they were produced.
The New York-based group said two attacks took place in the town of Kafrzeita, on April 11 and April 18 respectively, killing 2 people. The town of Temanaa was attacked on April 13 and 18, and six people died in those attacks. And an attack on the town of Telmans on April 21 killed 3.
There were no details as to whether there was any unusually fierce fighting in the area at the time. Associated Press interviews in late April with more than a dozen activists, medics and residents on the opposition side found consistent claims that chlorine gas was used in Kafrzeita.
Although chlorine canisters are widely-available and easy to use, Nadim Houry of Human Rights Watch said the government was the most likely perpetrator.
"This happened on multiple days, in different locations, and there is a consistency in what happened," Houry said. "That's what gave us that level of confidence to say there is strong evidence pointing in this direction."
Syrian forces frequently drop crude explosive-laden barrels onto rebel-held areas from helicopters, and opposition fighters do not have military aircraft.
HRW said half of the witnesses it spoke to reported seeing clouds of yellowish smoke as the bombs went off, which is consistent with chlorine canisters exploding.
Remnants of the canisters indicated they were dropped from heights, and the multiple attacks over several days, each with consistent testimony, strongly suggested eyewitness claims were true, the group said.
"I was less than one kilometer (half mile) away and saw the helicopter roaming and dropping the bomb," Human Rights Watch quoted an unidentified eye-witness from Telmans.
"I followed the dark yellowish smoke ... As soon as I arrived I smelled a horrible, strong smell. I started coughing and tears came from my eyes. People around me were suffocating."
Human Rights Watch underscored that the use of chlorine gas as a weapon is prohibited under international law.
"This is one more reason for the U.N. Security Council to refer the situation in Syria to the International Criminal Court," Houry said.
The Syrian government narrowly avoided Western-backed airstrikes after the August attack that killed hundreds of civilians. Instead, the Security Council ordered Syria to dismantle its chemical weapons infrastructure and destroy its arsenal, by June 30.
June is also the month of upcoming presidential elections. Assad is widely-expected to win the June 3 vote as his opponents are unlikely, unwilling or unable to vote.
There are two contenders running against him. One of them, Hassan al-Nouri said in an interview aired late Monday on state-run television that Syria is experiencing a "real and honest democratic experience," echoing Assad's previously expressed remarks on the balloting.
Syria's uprising began as mostly peaceful protests against Assad's rule in March 2011, but evolved into a civil war with sectarian overtones. Now in its fourth year, activists estimate 150,000 people have been killed so far in the conflict.
Islamic extremists, including foreign fighters and Syrian rebels who have taken up hard-line ideologies, are playing an increasingly prominent role in the war. But there has also been a backlash against the most extreme group, the al-Qaida breakaway group known as the Islamic State of Iraq and the Levant. Since January, rebel brigades, including its rival, the al-Qaida-linked Nusra Front, have clashed with the group.
On Tuesday, the Britain-based Syrian Observatory for Human Rights estimated that the infighting has killed around 6,000 people so far. The Observatory obtains its information from a network of activists on the ground.