Two Syria chemical weapons sites not yet inspected
In a statement, the Organisation for the Prohibition of Chemical Weapons (OPCW) said verification activities had been completed at 21 out of 23 sites.
"Efforts to ensure the conditions necessary for safe access to those sites will continue," it added.
On Sunday, Syria submitted to the OPCW a plan to destroy its chemical arsenal.
The Hague-based organisation said that it did so three days ahead of the 27 October deadline set by UN Security Council resolution 2118.
'Ceasefires needed'
Correspondents say the fact that inspectors have not been able to reach two of the 23 sites means that one of the first in a series of tight deadlines for the destruction or removal of Syria's chemical weapons stockpile by mid-2014 has been missed.
The OPCW has not said who was responsible for the security concerns, only that negotiations to ensure the conditions necessary for safe access to the two remaining sites will continue.
Officials say that ceasefires between government and rebel forces may be needed to reach them.
The OPCW's mission was set up following outrage at a chemical weapons attack on 21 August. Hundreds of people were killed when rockets containing the nerve agent sarin were fired at several suburbs of the capital, Damascus.
The United States and other Western powers blamed the attack on forces loyal to President Bashar al-Assad. But Mr Assad accused rebels of being behind it.
Sixty inspectors from the OPCW and UN have been in Syria since 1 October.
Under resolution 2118, Syria's chemical weapons production equipment must be destroyed by 1 November and stockpiles must be disposed of by mid-2014.
The organisation's work in Syria marks the first time the international chemical weapons watchdog - which won this year's Nobel Peace Prize - has been asked to oversee the destruction of a chemical arsenal during an armed conflict.
More than 100,000 people have been killed in the fighting that has ravaged Syria for two-and-a-half years, according to the UN.
A further two million people have fled Syria and some 4.5 million have been displaced internally.
BBC
