BEIRUT (AP) -- An international human rights group said Tuesday it has strong evidence that Syrian army helicopters dropped bombs carrying chlorine gas on three rebel-held towns last month.
The report by the New-York based Human Rights Watch adds to concerns that chemical weapons are still being used in Syria, eight months after a chemical attack killed hundreds of civilians in the suburbs of the capital, Damascus.
The Syrian government did not immediately comment on the HRW claims. It has accused rebels of using chlorine gas in the past.
The Organization for the Prohibition of Chemical Weapons, which monitors the implementation of the Chemical Weapons Convention, sent a team to Syria this month to investigate the chlorine claims.
OPCW spokesman Michael Luhan declined to comment on the Human Rights Watch report.
"We are there to do our own, independent fact finding mission," he said.
In its report, Human Rights Watch said 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, it said.
Chlorine gas in bombs is not very lethal, but HRW said it appeared to have been used to terrorize residents and cause widespread panic.
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 the factory where the canisters were produced.
The group said two attacks took place in the town of Kfar Zeita, on April 11 and April 18, and killed two people. The town of Temanaa was attacked on April 13 and 18, killing six people, it said. An attack on the town of Telmans on April 21 killed three, according to HRW.
There were no details as to whether there was any unusually heavy 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 Kfar Zeita.
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 gas.
Remnants of the canisters indicated they were dropped from a significant height, and the multiple attacks over several days, each with consistent testimony, 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.
Following the deadly chemical attack outside Damascus in August, the Syrian government averted U.S. airstrikes by agreeing to dismantle its chemical program. A joint U.N.-OPCW mission overseeing the removal of Assad's chemical arsenal said 92 percent of Syria's stockpile has been transported to Danish and Norwegian ships for destruction at sea. The entire stockpile is to be purged by the end of June.
Speaking aboard a Danish ship taking part in the mission, Denmark's foreign minister urged Syria to relinquish the last of its chemical agents.
"That is the clear message to the Syrian government that we expect them to deliver the last containers as soon as possible, hopefully within days," Lidegaard told The Associated Press aboard the Ark Futura as it lay at anchor in international waters off the coast of Cyprus.
Also Tuesday, Syria's main Western-backed opposition group welcomed a decision from France and Germany not to allow Syrians to vote remotely in Syria's June 3 presidential election -- a poll Assad is widely expected to win.
In a statement, the Syrian National Coalition called for other countries to take similar measures in an international boycott of the vote.
The opposition and its Western backers, including the U.S, France and Germany, have rejected the planned election in the middle of Syria's civil war as a sham.
Associated Press writers Ryan Lucas in Beirut, and Menelaos Hadjicostis aboard the cargo ship Ark Futura contributed to this report.
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