Deadly clashes raged on the edge of Damascus on Wednesday and rival rebel factions battled each other in northern Syria as international chemical weapons inspectors began to secure the sites where they will work.

The fighting underscored the immense security challenge that the dozens of disarmament experts must negotiate as they work amid the civil war to meet tight deadlines for eliminating President Bashar Assad's estimated 1,000-ton arsenal of chemical weapons.

The inspectors' mission — endorsed by a U.N. Security Council resolution passed last week — is to scrap Syria's capacity to manufacture chemical weapons by Nov. 1 and destroy its entire stockpile by mid-2014.

A convoy of SUVs with U.N. markings departed the central Damascus hotel where the team from the Organization for the Prohibition of Chemical Weapons is staying as the inspectors headed out for their first full day in the country.

The U.N. and OPCW said in a statement that "joint work with the Syrian authorities has begun on securing the sites where the team will operate, especially in outlying areas." It added that planning continues for disabling production facilities as do discussions on the size of Syria's stockpile.

One of the challenges the inspectors face is navigating the war itself.

On the northern edge of Damascus, fierce clashes between Syrian troops and al-Qaida-linked fighters killed at least 19 soldiers and pro-government militiamen in the past three days, according to the Britain-based Syrian Observatory for Human Rights.

The fighting in the contested district of Barzeh flared Monday when the army stepped up attacks against opposition forces who have been trying to capture the area for months, the Observatory said. Districts such as Barzeh, on the edge of Damascus, are important for rebels based in the capital's outer suburbs as the fighters try to move closer to the heart of the city.

The rebels, mostly from the ranks of al-Qaida-linked Jabhat al-Nusra fighters, also sustained losses but did not disclose them, the Observatory said. It also noted clashes in Jobar on the capital's eastern edge.

In northern Syria, the Observatory reported clashes between al-Qaida rebels and more moderate groups in the town of Azaz on the Turkish border. Militants from the Islamic State of Iraq and the Levant made advances against fighters from the Western-backed Free Syrian Army, the Observatory said. There were no reports of casualties.

The rebel infighting, which in recent months has risen in intensity, adds a new layer of complication to the 2½-year-old conflict, in which more than 100,000 have been killed.

Syria's war is the first that inspectors from the OPCW have faced in a disarmament mission. An advance group of 19 OPCW experts and 14 U.N. staff members arrived Tuesday in Damascus, and they will be joined within a week by a second group of inspectors.

Syrian Information Minister Omran al-Zoubi told The Associated Press that the country will cooperate with the OPCW and facilitate the experts' mission, "including destroying the (chemical) stockpile."

Experts at The Hague, where the OPCW is based, say the inspectors' priority is to reach the first milestone of helping Syria scrap its ability to manufacture chemical weapons by a Nov. 1 deadline. Some of the inspectors will double-check Syria's initial disclosure of what weapons and chemical precursors it has and where they are located, while others will begin planning the logistics for visits to every site where chemicals or weapons are reportedly located.

The inspectors' mission stems from a deadly attack on opposition-held suburbs of Damascus on Aug. 21 that the U.N. has determined included the use of the nerve agent sarin. The U.S. and its allies accuse the Syrian government of being responsible, while Damascus blames the rebels.

In the wake of the attack, the Obama administration threatened punitive missile strikes against the Assad regime, setting off weeks of intense diplomacy that culminated with the U.N. resolution adopted last week.

At the U.N., the Security Council called for immediate access in Syria to provide desperately needed humanitarian aid. The presidential statement urges the Syrian government to facilitate "safe and unhindered humanitarian access" to people in need.

A presidential statement is a step below a resolution. Some diplomats consider presidential statements legally binding, but others do not.