UN Secretary-General Ban Ki-moon has called for UN experts inside Syria to probe whether chemical weapons were used to be given "full access" to the sites of the alleged incidents.
After months of negotiations, a 20-member UN team arrived in Damascus on August 18 and started its work on August 19.
They are expected to investigate three sites, including a village just west of the embattled northern city of Aleppo.
Both Damascus and the rebels have accused each other of using chemical weapons.
Speaking to reporters on August 19, Ban said the confirmed use of chemical weapons inside Syria would "constitute an international crime."
UN Mideast envoy Robert Serry told the Security Council last month that the U.N. has received 13 reports of alleged chemical weapons use in Syria.
Meanwhile, Jordan announced on August 19 that it is preparing for the potential outbreak of chemical warfare across the border in Syria.
Prime Minister Abdullah Ensour told reporters in Amman that Jordan was bolstering its military presence and emergency preparedness in northern towns and villages along the 370-kilometer border.
Based on reporting by AP and dpa