"The number of sites visited where inspection activities have been conducted has increased to 14," the Organisation for the Prohibition of Chemical Weapons said in a statement.

Damascus has identified a total of 20 sites but the OPCW has not provided an exact number, saying only that "more than 20 sites" are affected by the ongoing inspection.

The Hague-based organisation said on Thursday that it had completed "nearly 50 percent" of the inspection of Syria's chemical arsenal, which has to be destroyed by mid-2014. A day earlier, it had said that 11 sites had already been inspected.

Despite the progress made, an OPCW political advisor on Syria told journalists Thursday that security remained a concern for the unprecedented mission in the war-torn country.