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WASHINGTON (Reuters) - The White House on Wednesday said it is concerned that international inspectors have received "credible allegations" that chemical weapons are still being used in Syria.

The government of Bashar al-Assad had pledged to hand over its chemical weapons stockpiles after the United States threatened military intervention in 2013 following sarin gas attacks that killed hundreds of residents in a Damascus suburb.

But international inspectors have found traces of sarin and VX nerve agent at a military research site in Syria that had not been declared to the global chemical weapons watchdog, the Organization for the Prohibition of Chemical Weapons.

"We're aware that the OPCW continues to receive credible allegations that the use of chemical weapons in Syria is still taking place," White House spokesman Josh Earnest told reporters in a briefing.

"Attempts by the OPCW to resolve some gaps and inconsistencies in Syria's declaration of their chemical weapons have gone unresolved," he said, noting the OPCW continues to investigate.

A man drives his motorcycle as he wears a gas masks after what activists said was a chlorine gas attack on Kansafra village at Idlib countryside, Syria May 7, 2015.STRINGER / Reuters