The team of 20 inspectors from The Hague-based Organisation for the Prohibition of Chemical Weapons is implementing a United Nations resolution passed last week that ordered the elimination of Syria's chemical arms.
The operation to rid Syria of chemical weapons by a target date of mid-2014 will be one of the largest and most dangerous of its kind.
The arsenal is believed to include more than 1,000 tonnes of sarin, mustard gas and other banned chemicals stored at an estimated 45 sites across the war-torn country.
An outgoing UN team of chemical arms experts has ended its second mission to Syria to probe seven alleged gas attacks. It hopes to present a final report by late October.
Earlier this month that UN team submitted an interim report that confirmed the use of the nerve agent sarin in August 21 attacks on the outskirts of Damascus.
The United States threatened military action in response, accusing forces loyal to President Bashar al-Assad of deliberately killing hundreds of civilians with rocket-delivered nerve agents.
Syria denied the allegations but agreed to relinquish its chemical arsenal, effectively heading off a strike, under a US-Russian deal which was enshrined in the landmark UN resolution.
The OPCW team arrived in Beirut on Monday before it crosses into Syria. It is unable to fly to Damascus because the road between the airport and the city is the scene of frequent fighting.
In his first comments since the UN resolution was passed, Mr Assad said on Sunday that his regime "will comply".
"History proves that we have always honoured all treaties we have signed," he told Italy’s RAI 24 television.
The UN arms resolution also calls for the convening of a much-delayed peace conference in Geneva as soon as possible, with Ban Ki-moon, the UN Secretary General, proposing a mid-November date.
The Syrian opposition reacted angrily to the resolution, arguing that it legitimised Mr Assad’s regime.
"This agreement is a disgrace for the United Nations, it's scandalous," said Qassem Saadeddine, a former colonel in the Syrian army who defected in 2012.
"The international community has grabbed on to chemical arms and forgotten the 100,000 victims of the conflict," said Saadeddine, who represents the military command of the rebel Free Syrian Army.
