Syria has said it will co-operate with the mission, set up after a US-Russia deal endorsed by the UN.

It is the first time the Organisation for the Prohibition of Chemical Weapons (OPCW) has been asked to destroy a country's chemical arms during a war.

Correspondents say the OPCW inspectors face a daunting task.

Syria's Foreign Minister Walid Muallem has said that seven out of the 19 chemical weapons sites declared by the government last month are in combat zones.

The BBC's Jim Muir in Beirut says it could be complicated for the inspectors to gain access to these areas; local truces may be needed to allow the work to proceed.

A spokesman for the opposition Syrian National Coalition, Monzer Akbik, told the BBC that the rebel Free Syrian Army (FSA) would ensure that inspectors "will be protected, and granted access to all locations".

However, the FSA is only one of several rebel factions operating in Syria, and its local commanders have often displayed a high degree of autonomy.

UN chemical weapons inspectors filed an interim report last month confirming that the nerve agent sarin had been used in an attack on the outskirts of Damascus on 21 August that killed hundreds of people.

Syria's chemical weapons arsenal is believed to include more than 1,000 tonnes of sarin, the blister agent sulphur mustard and other banned chemicals stored at dozens of sites.

Last month, it submitted to the OPCW a full account of its arsenal, as part of the US-Russian initiative that saw it accede to the Chemical Weapons Convention (CWC).

An OPCW official told the AFP news agency on Sunday: "At this point, we have absolutely no reason to doubt the information provided by the Syrian regime."
Logistics talks

The OPCW inspectors - based in The Hague - stayed overnight in Beirut, Lebanon, before crossing into Syria on Tuesday.

They were first expected to visit the foreign ministry in Damascus to discuss operational logistics before verifying the sites and making assessments.

The arms monitors are then expected to destroy the equipment used for mixing and preparing chemical weapons, as well as the munitions used to deliver them.

Under the agreement between the United States and Russia, this work should be finished by November. Some chemical stocks will be removed safely and destroyed outside Syria, while others will be collected up for destruction inside the country.