Syria flatly denied the allegations, and they have yet to be confirmed by any foreign country or international organization. But if true, they highlight the limitations of the global effort to rid President Bashar Assad's government of its chemical weapons.
Witnesses near Damascus and in a central rebel-held village told the AP of dozens of cases of choking, fainting and other afflictions from inhaling fumes that some said were yellowish and smelled like chlorine cleanser. Some of those interviewed said they believe the gas was responsible for at least two deaths.
They said the fumes came from hand grenades and helicopter-dropped "barrel bombs," which are crude containers packed with explosives and shrapnel.
Activists have posted videos similar, though on a far smaller scale, to those from last August's chemical weapons attack near Damascus that killed hundreds of people and nearly triggered U.S. airstrikes against Syria. The new footage depicts pale-faced men, women and children coughing and gasping at field hospitals.
The U.N. Security Council called for an investigation Wednesday. Council members expressed "grave concern" over the allegations, said Nigeria's U.N. Ambassador U. Joy Ogwu, council president.
It's an accusation that carries high stakes, and the Syrian opposition has an interest in pushing such claims in hopes of spurring the world to take stern action against Assad, who has been locked in a civil war for three years and faces a Sunday deadline for handing over all his chemical weapons for destruction.
Chlorine is a potentially lethal chemical with a multitude of ordinary civilian uses, including laundry bleach and swimming-pool disinfectant. In high concentrations, it can attack the lungs and asphyxiate victims.
While chlorine was first deployed on the battlefield in World War I, it is no longer officially considered a warfare agent and is not among the chemicals declared by Syria. It is not as effective at killing as sarin - the nerve agent that was apparently used last summer - and experts say it is difficult to achieve high concentrations of chlorine by dropping it from the air.
Still, any toxic chemical is considered to be a chemical weapon if used for military purposes. Consequently, Syria's use of chlorine-filled bombs, if confirmed, would be a violation of the chemical weapons treaty that Assad's government signed last year as part of a deal to hand over its stockpile.
On Wednesday, Syria's U.N. Ambassador Bashar Ja'afari said his government categorically denied the use of chlorine gas. Ja'afari further disputed that chlorine gas could be categorized as a chemical weapon, saying "it is a mundane substance used for bleaching clothes in the laundry or disinfecting swimming pools."
U.S. State Department spokeswoman Jen Psaki said Monday that officials were still trying to determine what happened. On Sunday, French President Francois Hollande told Europe 1 radio station there were "elements" suggesting recent use of chemical weapons, but no proof.
Both countries bluntly accused the Syrian government of using sarin against civilian areas in the August attack near Damascus.
"I can understand the reluctance to undertake any firm action right now because the big priority is to get the other chemicals out of the country," said Jean Pascal Zanders, an independent chemical weapons consultant and disarmament expert. "Once these are out of the country, we can probably see a completely different dynamic with regards to Syria emerge. People will be less deferential to the Assad regime."
Zanders, who remains skeptical about the claims emerging from Syria pending more proof, said nobody wanted to upset the Assad government to the point that it would cease all cooperation, particularly with the relationship between the U.S. and Russia strained because of the Ukraine crisis.
Russia was a main sponsor of the deal to strip Syria of its chemical weapons. Syria has shipped out 86 percent of its declared stockpile so far, according to the Organization for the Prohibition of Chemical Weapons, the watchdog agency overseeing the process.
Syrian opposition forces have accused the government of using small amounts of poisonous gas over the past few months in several incidents affecting more than 100 people.
The Violation Documentation Center, a Syrian group that tracks human rights violations, issued a detailed report last week in which it claimed to have documented the use of chemicals in 15 instances since the beginning of the year in suburbs of Damascus, in Hama and in Idlib. The main Western-backed opposition group, the Syrian National Coalition, said it identified at least nine cases in recent months where the government used poison gas.
The most serious episode appears to have occurred in Kfar Zeita, a rebel-held village in Hama province some 200 kilometers (125 miles) north of Damascus.
Three activists and a medic gave similar accounts of how several bombs containing a chlorine-smelling gas were dropped on the village of some 20,000 people starting on April 11, triggering severe coughing, muscle contractions and choking.
"It smelt like eggs, then after a while it became like chlorine," Muaz Abu Mahdi, a Kfar Zeita activist who filmed a falling bomb, said in a Skype interview. He said it killed a girl and an elderly man.
He said he saw dozens of stricken people at a field hospital.
"They were lying on the ground of the clinic. ... Most of them had fainted. Others were shaking, and they couldn't flex their muscles. Others woke up dizzy. Others were coughing blood," Abu Mahdi said.
Adham Raadoun, a journalist working for a Syria-based opposition news network who lives on the edge of Kfar Zeita, said the bombs were dropped on residential areas. He said they released a yellowish smoke and smelled like chlorine cleanser.
Videos posted by activists showed rooms full of men, women and children who appeared to have serious breathing problems and were being fed oxygen by medics. One man lay on the floor, choking, as a medic rubbed his chest.
The videos corresponded with AP reporting on the incident in Kfar Zeita, although it could not be established what caused the symptoms.
Four activists near Damascus said Syrian forces had also used small amounts of poison gas in at least four incidents in clashes in rebel-held towns around the capital since December. They said it was typically packed in grenade-style weapons that could be hurled into rebel hideouts.
Syria's government accused the al-Qaida rebel group called the Nusra Front of releasing the chlorine gas in Kfar Zeita.
But some experts said Assad's forces are most likely responsible, because of the reports of canisters dropped from helicopters. The rebels are not known to have military aircraft.
"You can never be certain, but it's pretty close to certain that it was the Syrian military," said Paul Walker, who works for Green Cross International on chemical weapons disarmament. He added, however, that chlorine is easily obtained and easy to use.
"You can just open it and leave it blowing in the right direction," he said.
Zanders said very high concentrations would be needed to kill, something not easily achievable through barrel bombs dropped from helicopters. He said chlorine as such would not have a major effect on the battlefield but could be used to terrorize the population.
OPCW spokesman Michael Luhan said the watchdog group could not get into verifying the claims without a formal request from a government entity with credible information.
"So far, no state party has asked for an investigation," he said.
Associated Press writer Michael C. Corder in The Hague, Netherlands, contributed to this report.