A new Human Rights Watch report, the first of its kind since Syria agreed last summer to destroy its chemical weapons stockpile, says the Assad regime may have used chlorine gas in attacks just last month.

French Foreign Minister Laurent Fabius echoed the charge during a visit to Washington Tuesday, Reuters reported.

"We have at least 14 indications that show us that, in the past recent weeks again, chemical weapons in a smaller scale have been used, in particular chlorine," Fabius told a news conference through an interpreter. "Right now we are examining the samples that were taken."

President Barack Obama met on Tuesday with Syrian opposition leaders at the White House, telling them President Bashar al-Assad's government has lost legitimacy and that he supports their goal of a transition of power. Syrian Opposition Coalition President Ahmad Jarba thanked Washington for $287 million in nonlethal assistance and more than $1 billion in humanitarian aid for the region.

Throughout the three-year-old war in Syria, opposition activists have repeatedly charged that the government is using chemical weapons against opponents. 

The worst known chemical attack occurred Aug. 21, 2013, in the town of Ghouta. More than 1,400 people were reported to have been killed by nerve gas, including children. Under strong international condemnation, the Syrian government agreed to allow international observers into the country and to destroy its chemical stockpile, which was then one of the largest in the world.

The Human Rights Watch report, released Tuesday, says the regime's helicopters may have dropped barrel bombs with cylinders of chlorine gas on towns in the northern part of the country last month. Chlorine gas is a chemical agent and its use during wartime is prohibited under international law. But Human Rights Watch indicated that it could not independently verify the attacks. 

The two major groups of chemical weapons are nerve agents, which affect the nervous system, and toxins that burn. Nerve gas, also known as sarin, and mustard gas, are classified by experts as the most dangerous chemical weapons in the world. Chlorine, on the other hand, is not. According to Peter Walker, director of the Green Urbanism program at Global Green, chlorine is not considered deadly unless an individual ingests or inhales a large amount.

Chlorine was used as a weapon for the first time by the Germans in World War I almost 100 years ago. It serves multiple functions, both legitimate and banned, according to chemical weapons experts. 

“In general chlorine is a very common and is much less lethal than some of the agents we have talked about before in regard to Syria. The bad news is there is a lot of it around,” said Greg Thielmann, a senior fellow at the Arms Control Association and a former foreign service officer.

Though it is difficult to verify information in an increasingly restricted war zone, Thielmann said the chlorine was most likely stored in canisters that were inside the barrel bombs. Once the bombs dropped, they cracked and released the poison gas. Human Rights Watch noted the same process in its report.

Although chlorine is usually not used in warfare to kill, it can blind or cause lasting respiratory damage.

According to the chemical weapons experts interviewed, there are two ways to verify if the gases used in Syrian attacks involved chlorine: forensic and scientific. First, a professional team would have to analyze the remaining pieces of the bombs and evaluate the canisters. Second, the team would have to run lab tests for chlorine agents and examine victims who were experiencing symptoms of ingesting the chemical.  

Yet even if these steps were taken by a professional team, Thielmann said it would still be difficult to determine if the agent used in the attack was chlorine.

“By blood test you can determine exposure to sarin, but significant exposure to chlorine is less definite through blood tests,” he said.