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 the United States and France, 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.
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.
Syrians living outside the country will vote on May 28 and candidates can begin registering from today until May 1.
The United Nations condemned the announcement, warning it would torpedo efforts to reach a negotiated 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, referring to an agreement on a transition to democracy.
Ahmed Jarba, head of the opposition National Coalition, rejected the planned election as a "farce."
"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," his office said.
Syria's conflict began as a peaceful protest movement demanding democratic reform, but descended into war after Assad's regime unleashed a brutal crackdown on dissent.
On Tuesday, fighting raged in flashpoints across the country, while the air force struck rebel areas of Aleppo city, keeping up an aerial offensive the regime launched in December.
Jihadist sources meanwhile said German rapper-turned-militant Denis Mamadou Cuspert, who performed under the name Deso Dogg but took on the name Abu Talha al-Almani in Syria, was killed in a suicide bombing on Sunday carried out by rival fighters.