A team of UN chemical weapons experts arrived in Beirut on Wednesday en route back to Syria for further investigation into use of the banned arms, an airport source told AFP.
The team, led by chief expert Ake Sellstrom, is expected to examine the alleged use of chemical weapons some 14 times in Syria's 30-month conflict.
The team went to Syria last month and concluded in a report presented on September 16 that banned chemical weapons had been used on a wide scale.
There was clear evidence that sarin gas was used in an attack in the Eastern Ghouta neighbourhood near Damascus on August 21, the report said.
Sellstrom pointed out that the report was only an interim document, and that other allegations needed to be looked into.
"There have been other accusations presented to the UN secretary general, dating back to March, against both sides" in the conflict, he told AFP earlier this month.
There were "13, 14 accusations" that "have to be investigated".
He said the team hoped to be able to present a final report addressing all of the accusations "possibly by the end of October".
The August 21 attack, which the Syrian opposition and some parts of the international community blame on the regime, prompted Washington to threaten military action against Damascus.
The Syrian government denies using chemical weapons against its people, and has agreed to a US-Russian plan that will see it hand over its chemical arsenal for destruction.
The deal headed off US military action, but regime ally Russia is still wrangling with Britain, France and the United States over the wording of a UN resolution enshrining the accord.