A team of international disarmament experts is due to arrive in Damascus to begin work on dismantling Syria’s chemical weapons arsenal.
Syria says it will co-operate with the mission set up after a US-Russia deal endorsed by the UN Security Council.
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 19 declared chemical weapons sites 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.The OPCW experts filed an interim report last month confirming that the nerve agent sarin had been used in attacks on 21 August near Damascus.
Syria’s chemical weapons arsenal is believed to include more than 1,000 tonnes of sarin, mustard gas and other banned chemicals stored at dozens of sites.
Last month, it handed the OPCW a full account of its arsenal, as part of the US-Russian plan.
An official for the organisation 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 will first discuss operation logistics at the Foreign Ministry in Damascus 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.
A Free Syrian Army fighter fires his weapon towards snipers loyal to Syria’s President Bashar al-Assad in the Aleppo district of Salaheddine, 30 September, 2013.
OPCW disarmament teams face a large and dangerous operation in the midst of fierce fighting
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.