The group, led by chief expert Ake Sellstrom, flew to Beirut in Lebanon and travelled by overland convoy via the Masnaa border post to Damascus.
US President Barack Obama yesterday demanded tough Security Council action against Syria as the conflict there dominated debate at the annual UN General Assembly.
Sellstrom's team is expected to examine the alleged use of chemical weapons some 14 times in Syria's 30-month conflict that is estimated to have killed more than 110,000 people.
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.
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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 war, he told AFP earlier this month.
Last month's attack, which the Syrian opposition and some parts of the international community blame on the regime, prompted Washington to threaten military action against Damascus.
Assad's government denies using chemical weapons against its own 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 Syrian regime ally Russia is still wrangling with Britain, France and the United States over the wording of a UN resolution enshrining the accord.