The United States has indications that a toxic chemical, probably chlorine, was used in Syria this month and is examining whether the Syrian government was responsible, the U.S. State Department said on Monday, April 21, according to Reuters.

"We have indications of the use of a toxic industrial chemical" in the town of Kfar Zeita, State Department spokeswoman Jen Psaki said, referring to a rebel-held area.

"We are examining allegations that the government was responsible," she told a regular news briefing. "Obviously there needs to be an investigation of what's happened here."

Syrian opposition activists reported that helicopters dropped chlorine gas on Kfar Zeita on April 11 and 12. The U.S. ambassador to the United Nations, Samantha Power, told ABC television's "This Week" on April 13 that the attack was "unsubstantiated."

Psaki said chlorine was not one of the priority one or two chemicals Syria declared to the Organization for the Prohibition of Chemical Weapons (OPCW) under a Russian-U.S. agreement for the destruction of Syria's chemical weapons stockpile.

Psaki said the United States was still trying to determine the facts.

"We take all allegations of the use of chemicals in combat use very seriously," she said." We'll work with the OPCW, who is obviously overseeing the implementation, and determine if any violation occurred."

A UN inquiry found in December that sarin gas had likely been used in Jobar, on the outskirts of Damascus, in August and in several other locations, including in the rebel-held Damascus suburb of Ghouta, where hundreds of people were killed.

The Ghouta attack caused global outrage and a U.S. threat of military strikes that was dropped after Syrian President Bashar al-Assad pledged to destroy his chemical weapons arsenal.

The Syrian government failed to meet a February 5 deadline to move all of its declared chemical substances and precursors, some 1,300 metric tons, out of the country. It has since agreed to remove the weapons by late April.