"We have indications of the use of a toxic industrial chemical" in the town of Kfar Zeita, State Department spokeswoman Jen Psaki said on Monday, Reuters reported.

"Obviously there needs to be an investigation of what's happened here."

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-US agreement for the destruction of Syria's chemical weapons stockpile.

Psaki claimed 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 Damascus suburb of Ghouta, where hundreds of people were killed.

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

The Syrian government has repeatedly rejected allegations of using chemical weapons, blaming foreign-backed militants for such attacks.

BA/BA