BEIRUT — International inspectors overseeing the destruction of Syria’s chemical weapons stockpile have missed an early deadline in a brutally tight schedule after security concerns prevented them from visiting two sites linked to Damascus’ chemical program.

Experts from the Organization for the Prohibition of Chemical Weapons were to have checked all 23 of Syria’s declared chemical sites by Sunday, but the organization said Monday that inspectors have visited only 21 because of security issues. While there are no consequences for missing the deadline, the group’s failure to meet it underscores the ambitious timeline as well as the risks its inspectors face in carrying out their mission in the middle of Syria’s civil war.

The OPCW did not say who was responsible for the security problems, but the organizations’ director-general has said in the past that temporary cease-fires may have to be negotiated between rebels and forces loyal to President Bashar Assad to reach some sites. The chemical weapons watchdog said it has not given up hope of gaining access to the two locations.