Their mission stems from a deadly Aug. 21 attack on opposition-held suburbs of Damascus in which the U.N. has determined the nerve agent sarin was used. The U.S. and its allies accuse the Syrian government of being responsible for the attack, while Damascus blames the rebels.
The Obama administration threatened to launch punitive missile strikes against Syria, prompting frantic diplomatic efforts to forestall an attack. Those efforts concluded with last week's unanimous U.N. Security Council resolution endorsing the elimination of Syria's chemical weapons.
The mid-2014 deadline the tightest OPCW inspectors have faced is only one of several challenges the inspectors must overcome. Another major hurdle is purging Assad's estimated 1,000-ton arsenal in the middle of Syria's bloody civil war.
The conflict, which is rooted in what began as peaceful protests in March 2011, has laid waste to the countries' cities, shattered its economy and driven more than 2 million people to seek shelter abroad. The violence affects every corner of Syria, which has become a patchwork of rebel-held and regime-held territory.
In recent months, an outburst of infighting among the myriad rebel groups opposed to Assad added another layer to an already complicated conflict.
Al-Qaida-linked fighters in Syria have been some of the most effective forces on the battlefield, and they often fight alongside more moderate rebel brigades against government troops. But with growing frequency, Islamic extremist fighters and more mainstream rebels also are turning their guns on each other. Turf wars and retaliatory killings have evolved into ferocious battles that have effectively become a war within a war in northern and eastern Syria, leaving hundreds dead on both sides.
One of the worst instances broke out late last month in the town of Azaz near the Turkish border as the al-Qaida-affiliated Islamic State in Iraq and the Levant battled the more moderate Northern Storm Brigade linked to the Western-backed opposition.
In a statement released late Wednesday, six rebel groups urged ISIL and the Northern Storm Brigade to "cease fire immediately" and resolve their differences before an Islamic court. The appeal also called on the al-Qaida-linked ISIL to withdraw its fighters to areas where they were before the clashes in Azaz erupted late last month.
The appeal's signatories included the Islamic Army, the Tawheed Brigade and Ahrar al-Sham three of the most powerful rebel groups.
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Associated Press writer Barbara Surk contributed to this report.