A team of international disarmament experts has arrived in Syria to begin work on dismantling the country's stockpile of chemical weapons.
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."
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.
All this material is supposed to have been disposed of by the target date of the middle of next year.
Syria's President Bashar al-Assad has promised to comply with the disarmament deal. "History proves that we have always honoured all treaties we have signed," he said in an interview with Italian television on Sunday.
Russia and America are in the process of destroying their own chemical arsenals. This process has taken years longer than expected.
Washington, Moscow and others are hoping to build on the rare consensus achieved over the chemical weapons issue, to push for peace settlement talks in Geneva. The UN Secretary General Ban Ki-moon has proposed a date in mid-November for the discussions.
But correspondents say many obstacles remain to be overcome before credible and serious negotiations can take place.
Also on Tuesday, campaign group Human Rights Watch reported that a secondary school in the rebel-held town of Raqqa had been hit on 29 September by "a Syrian government airstrike using fuel-air explosive bombs", killing 14 civilians.
Twelve of the dead were students attending their first day of classes, according to the group.
The group has also documented the use of incendiary devices suspected to contain substances like napalm elsewhere in the conflict.
Like fuel-air explosive bombs, the weapons allegedly used in the Raqqa attack are not classified as chemical weapons but rights groups have condemned their use in populated areas as causing widespread civilian casualties.