Syria’s President Bashar al-Assad (L) shakes hands with Russia’s Deputy Foreign Minister Sergei Ryabkov before a meeting in Damascus in this June 28, 2014.

The head of the international effort to rid Syria of its chemical weapons said Monday the next step is investigating possible discrepancies in Syria’s declaration of its stockpile, as well as the destruction of its production facilities.

Sigrid Kaag briefed the U.N. Security Council by videoconference less than a week after the last of hundreds of tons of chemical weapons declared by Syria were loaded onto a U.S. cargo ship for destruction at sea.

“There is still a lot of outstanding business, including the destruction of production facilities, hangers, garages, etc., that need to be destroyed,” Britain’s U.N. Ambassador Mark Lyall Grant said as he went into the briefing. “There is also the question of the discrepancies in the original declaration, and that is something that we are pursuing with (the Organization for the Prohibition of Chemical Weapons) and that will be raised again with OPCW this week.”

Syria’s mission to the United Nations did not immediately respond to an email seeking comment.

Western diplomats said Kaag, head of the joint U.N.-OPCW mission, told them that technical experts from the OPCW were talking to Syria about discrepancies between the weapons the country has acknowledged possessing and what the evidence on the ground has shown.

The diplomats spoke on condition of anonymity because they were not allowed to discuss the closed briefing publicly.

Syria agreed to surrender its arsenal last fall when the U.S. threatened missile strikes in retaliation for a chemical attack on a rebel-held suburb of Damascus. The attack is believed to have killed more than 1,000 people.

Last week, the last 1,300 tons of the arsenal Syria has declared were transferred onto the U.S. cargo vessel MV Cape Ray. The ship moved into international waters and on Monday began the two-month process of destroying the chemicals.

The material handed over by Syria included mustard gas and precursors to the nerve gas sarin. But questions remain over whether Syrian President Bashar al-Assad is hiding undeclared poison gases or attacking rebels with chlorine – a toxic industrial gas that is not specifically classified as a chemical weapon.

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