Western governments are growing impatient with Syria’s failure to follow up promptly on a first small shipment of chemical weapons and fear Damascus will miss a deadline to hand over all toxins by mid-2014.

Sources at the Organisation for the Prohibition of Chemical Weapons (OPCW), which is jointly overseeing the destruction process with the United Nations, said the concerns have been raised during internal discussions, but have not yet been reported to the U.N. Security Council.

Syria agreed to dismantle its entire chemical weapons program by June 30, under a deal proposed by Russia and agreed with the United States. It has until March 31 to relinquish around 500 tonnes of the worst substances, including more than 20 tonnes of mustard gas stored in liquid form.

That deadline had already been expected to slip, but the concern now is that the entire destruction program will be pushed back. Syria says the program faces security concerns.