New claims have emerged that President Bashar al-Assad's regime may have launched attacks with an industrial chemical earlier this month, despite an international agreement to eliminate Syria's chemical arsenal.
The latest evidence, cited by US and French authorities, comes as Syria plans to hold a June 3 presidential poll, which the United Nations and the Syrian opposition have slammed as a "farce" that flies in the face of efforts to end the country's three-year war.
"We have indications of the use of a toxic industrial chemical, probably chlorine, in Syria this month, in the opposition-dominated village of Kafr Zita," White House spokesman Jay Carney said Monday.
"We are examining allegations that the government was responsible."
The revelation follows Sunday's announcement by French President Francois Hollande that his country had "information" -- but no proof -- that Assad's regime was still using chemical weapons.
There have been conflicting accounts of an alleged chlorine gas attack in opposition-held Kafr Zita in the central Hama province earlier this month, with the government and the opposition trading blame.
Activists have also reported other chlorine gas attacks, most recently on Monday in the northwestern Idlib province.
The Organisation for the Prohibition of Chemical Weapons and other experts have spent months working to remove Syria's chemical stockpiles, following an agreement reached after deadly chemical attacks near Damascus last August that killed hundreds.
Western nations blamed those attacks on the Assad regime and the United States threatened military action before backing down and reaching a deal with Russia to eliminate the chemical weapons.
The OPCW said last week that 65 percent of Syria's stated chemical weapons have been removed from the country.
Although chlorine is a toxic chemical, it is widely used for commercial and domestic purposes, so Syria was not required to submit its stockpiles to the OPCW, a chemical weapons expert told AFP.
"However, as a chemical weapon it is prohibited under the Chemical Weapons Convention," which Syria joined last year, said Hamish de Bretton-Gordon, CEO of SecureBio, a British chemical weapons consultancy.
"The opposition could very easily get ahold of chlorine... However the delivery method that we've seen -- the use of helicopters -- I am certain the opposition don't have any helicopters."
He also said that although chlorine is a weak agent, its use would be "very effective in this kind of warfare, in urban, built-up areas, as chemical weapons find their ways into the nooks and crannies."
Syria meanwhile announced Monday that it will hold a June 3 presidential election, expected to return Assad to office.
Syria's first presidential election -- after constitutional amendments scrapped a referendum system -- is to go ahead despite violence which has killed more than 150,000 people since March 2011.
Speaker Mohammad al-Lahham announced the date in parliament, saying Syrians living outside the country would vote on May 28 and candidates would be able to register from Tuesday until May 1.
Voting would be "free and fair... and under full judicial supervision," he said.
However, the United Nations condemned the announcement, warning it would torpedo a political resolution of the conflict.
"Such elections are incompatible with the letter and spirit of the Geneva communique," UN spokesman Stephane Dujarric said in New York.
He was referring to an agreement on a transition to democracy as the basis for negotiations between the government and the opposition.
The opposition, which insists Assad step down and play no role in Syria's future, rejected the election as nothing more than a "farce."
"The Assad regime's announcement today that a 'presidential election' would be held in June should be treated as a farce," said the office of opposition National Coalition leader Ahmad Jarba.
"With vast parts of Syria completely destroyed by Assad's air force, army and militias over the last three years, and with a third of Syria's population displaced internally or in refugee camps in the region, there is no electorate in Syria in a condition to exercise its right to vote."
Syria's conflict began as a peaceful protest movement demanding democratic reform, but descended into war after Assad's regime unleashed a massive crackdown on dissent.
Half of the population has been forced to flee their homes, and more than nine million people need humanitarian assistance.