U.S. officials say they have indications that toxic chemicals were used in a rebel area of Syria this month.
White House spokesman Jay Carney said Monday that the alleged attack in the western village of Kafr Zita involved an industrial chemical that was probably chlorine.
"We are examining allegations that the government was responsible. We take all allegations of the use of chemicals in combat very seriously. And we are working to determine what happened. We will continue consulting and sharing information with key partners, including of course at the OPCW."
The Organization for the Prohibition of Chemical Weapons is working with the United Nations to rid Syria of its declared stockpile of such weapons. The OPCW said this week that 80 percent of those chemicals have been removed from the country or destroyed.
U.S. State Department spokeswoman Jen Psaki said Monday that chlorine was not on the list of chemicals that Syria declared last year when it agreed to destroy the arms under international pressure.
Washington and its allies say Mr. Assad's forces unleashed sarin gas last year, killing hundreds of civilians. The Syrian government said it was the opposition fighters who used the chemical weapons.
French President Francois Hollande says France also had indications that chemical weapons are still being used in Syria.
Meanwhile, the United Nations is expressing concern about Syria's plans to hold a presidential election on June 3.
Stephane Dujarric, a spokesman for U.N. Secretary-General Ban Ki-moon, told reporters Monday that the vote goes against an international outline for bringing a political solution in Syria.
"Both the secretary-general and the joint special representative, Lakhdar Brahimi, have repeatedly warned that the holding of elections in the current circumstances, amid the ongoing conflict and massive displacement, will damage the political process and hamper the prospects for the political solution that the country so urgently needs. Such elections are incompatible with the letter and spirit of the Geneva communique. We will nonetheless continue to search and build upon any opening for a solution to the tragedy in Syria."
The 2012 Geneva document calls for a transitional government for Syria, which would then hold fresh elections.
The election that the government announced Monday will give President Bashar al-Assad the chance to win a third seven-year term in office. The opposition trying to oust him from power immediately dismissed the vote as a farce.
Mr. Assad has been in power since 2000, when he took office following the death of his father, who ruled Syria for 30 years.
The president won his second term in 2007, taking 97 percent of the vote in an election boycotted by the opposition. He was the only candidate on the ballot.
Mr. Assad has been battling rebels in a conflict that began as peaceful protests in March 2011 and quickly grew into a civil war that has killed more than 150,000 people, mostly civilians.
Another 2.6 million people have fled Syria to surrounding countries. The war has displaced more than 6.5 million people within Syria.