Ariel Schalit, Associated Press
Israeli soldiers drive armored personal carriers during a military exercise in the Israeli-controlled Golan Heights near the border with Syria, Wednesday, Oct. 2, 2013.

BEIRUT — Six rebel groups called on two major rival factions battling each other in northern Syria to end the infighting as international inspectors tasked with overseeing the destructions of the government's chemical arsenal pressed on with their second full day of work in the country on Thursday.

The inspectors' mission — endorsed by a U.N. Security Council resolution last week — is to scrap Syria's capacity to manufacture chemical weapons by Nov. 1. and to destroy the entire stockpile of President Bashar Assad's estimated 1,000-ton arsenal of chemical weapons by mid-2014.

They are working against the backdrop of a relentless civil war that has pitted a wide array of opposition fighters and groups against Assad's troops and pro-government militiamen in all major cities and their surroundings.

The conflict, now in its third year, has recently become even more complex, with rebel groups turning their guns on one another — particularly in the north where opposition fighters are now battling over territory they captured together from government troops in the past year.

The rebel infighting underscores the immense security challenges that the international weapons experts face as they work amid the civil war to meet tight deadlines.

In a statement released late Wednesday, six rebel groups urged the two major rebel factions battling each other around the town of Azaz near the border with Turkey to "cease the fire immediately" and resolve their differences before an Islamic court.

The two major factions are al-Qaida's Islamic State in Iraq and the Levant on one side, and the Western-backed Free Syrian Army's Northern Storm Brigade on the other.

According to the Britain-based Syrian Observatory for Human Rights, which documents the civil war and also the rebel-on-rebel fighting, among those that signed the appeal are the Islamic Army, the Tawheed Brigade and Ahrar al-Sham group, all affiliated with the Western-backed FSA alliance.

A convoy of three U.N. vehicles left from a hotel in central Damascus on Thursday with nine experts from the Netherlands-based chemical weapons watchdog, but it was not clear where they were heading. The team now consists of an advance group of 19 experts from the Organization for the Prohibition of Chemical Weapons experts and 14 U.N. staff members who arrived on Tuesday. A second group of inspectors is to join them within a week.

Their mission stems from a deadly Aug. 21 attack on opposition-held Damascus suburbs 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 for the attack. The U.S. has said it killed 1,400 people. Death toll estimates by activists and rights groups are significantly lower, but still in the hundreds.

The Syrian conflict began in March 2011 as largely peaceful protests against Assad's rule, then gradually turned into an uprising and a civil war that has killed at least 100,000 people so far. More than two millions Syrians have been displaced, many fleeing to neighboring Lebanon, Jordan, Turkey and Iraq.

Also Thursday, an international human rights organization released a report saying that according to its estimate, Assad's government is unlawfully holding tens of thousands of regime opponents and torturing many in custody.

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Those arrested include medics who treated wounded protesters, businessmen who raised money for displaced Syrians and even software developers who worked with citizen journalists, said Human Rights Watch.

The New York-based group offered accounts of 21 Syrians who had been detained and who said they were beaten in custody with batons, cables and metal rods. The report also cited some as saying they were sexually abused and raped in custody.


Associated Press writer Bassem Mroue in Beirut contributed to this report.