UN chemical weapons experts, in Syria to investigate the alleged use of the banned arms, left their Damascus hotel on Sunday to conduct a new mission, an AFP photographer said.

The experts, who arrived in the Syrian capital on Wednesday for their second visit in two months, headed out on board four UN vehicles.

The United Nations has said the experts are investigating seven alleged chemical weapons attacks and expect to wind up their work on the ground on Monday.

They hope to have a comprehensive report ready "by late October".

The UN mission is separate from a team of inspectors from the Organisation for the Prohibition of Chemical Weapons due to start work in the coming days to destroy Syria's chemical weapons arsenal.

The alleged attacks being probed include a March 19 incident in Khan al-Assal, in Aleppo province in northern Syria, that was reported by both the government and rebels, each accusing the other of responsibility.

Other sites being investigated include the Sheikh Maqsud neighborhood of Aleppo city, allegedly the site of an attack in April, and Saraqeb in the northwest province of Idlib, on the border with Turkey.

The team is also continuing investigations into an attack on Aug 21 in the suburbs of Damascus that coincided with their first mission to Syria last month.

The UN experts visited the site of that alleged attack during their first trip and have issued a preliminary report confirming the use of the nerve agent sarin in the area.

In addition, the team is investigating three subsequent alleged attacks - on Aug 22 in Bahhariyeh, near Damascus, on Aug 24 in the Jubar district of the capital, and a seventh incident reported in Ashrafiyeh Sahnaya, Damascus province, on Aug 25.

"In the course of performing their task, the experts have received several documents and samples and have conducted many interviews," the UN said in a statement.

United Nations vehicles carrying chemical weapons experts leave a hotel in the Syrian capital Damascus on Sunday. Louai Beshara / Agence France-Presse