The Nobel Peace Prize-winning group tasked with inspecting Syrian chemical weapons sites and destroying production equipment may miss the first major deadline of its mission.

The Organization for the Prohibition of Chemical Weapons says it has visited 21 of 23 sites disclosed by the Syrian government but security concerns have kept them away from the final two and a ceasefire may be necessary for them to access the sites, the Wall Street Journal reports.

It's not clear where the final two sites are, but experts believe they are in the town of Safira, in a heavily contested area near Aleppo. The facilities there are believed to be the country's largest and most modern. United Nations resolution 2118 calls for Syria's chemical weapons production equipment to be destroyed by midnight Friday and stockpiles to be disposed of by mid-2014 and UN chief Ban Ki-moon says that despite the snag, the mission is on course and the Syrian regime "has extended consistent, constructive cooperation," reports Al Jazeera.