Chemical Weapon inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) - the international chemical weapons watchdog - have completed verification activities at 21 of the 23 sites. The other two sites are located in conflict zones where conditions necessary for safe access could not be ensured. The U.S., however, remains skeptical on the actual number of chemical weapon sites, say reports.

"The two remaining sites have not been visited due to security reasons. Efforts by the Joint Mission to ensure the conditions necessary for safe access to those sites will continue," said a brief press release issued by the OPCW.

Media reports say the two sites are in rebel-held or contested areas. Reports confirm that at least one of them is in the town of al-Safira, which has seen intense conflict for months. Experts believe that many rebels in the area belong to al-Qaida-linked groups.

The fact that inspectors were unable to visit the two sites, means that they have missed the first in a series of tight deadlines, reports BBC.

Syria's chemical weapons arsenal needs to be destroyed or removed by mid-2014. Observers, however, appreciate the fact that the OPCW, the 2013 Nobel Peace Prize winner, is working in a conflict-zone for the first time.

Even as the work of the inspectors' progresses, the U.S. Administration continues to be skeptical about the actual number of weapon sites in Syria. The U.S. believes that there are at least 45 sites involved.

The New York Times, meanwhile said, that Syria has now provided information on the existence of 41 chemical weapons facilities at the 23 sites it disclosed. These details could help close the gap between Syria's disclosure and U.S. estimation, says the newspaper.

Accordingly to UN figures, the two-and-a-half year long conflict in Syria has seen over 100,000 people killed, two million people rendered refugees aboard and some 4.5 million people displaced internally.