UN chief Ban Ki-moon described the mission to identify who is behind the chemical attacks in a seven-page letter to the Security Council.

Ban will await the council's go-ahead before launching a recruitment drive for top experts to carry out the Syria investigation.

Earlier this week, Doctors Without Borders (MSF) said it had treated civilians suffering from apparent exposure to a chemical agent in Marea, a town near the northern city of Aleppo, following an attack last week.

The Syrian American Medical Society (SAMS) said its own doctors had identified the agent as mustard gas.

Reports emerged almost two weeks ago that "Islamic State" (IS) jihadists in Iraq may have used mustard gas against Iraqi Kurdish fighters.

"The continuing reports of the use of chemical weapons, as well as the use of toxic chemicals as a weapon in the Syrian conflict are deeply disturbing," said Ban in a separate statement Thursday.

"The international community has a responsibility to hold the perpetrators accountable and to ensure that chemical weapons never be used again as an instrument of warfare," he said.

The UN Security Council unanimously endorsed a resolution to set up a joint investigative mechanism earlier this month.

The future investigation would seek "to identify to the greatest extent feasible individuals, entities, groups or governments who were perpetrators, organizers, sponsors or otherwise involved in the use of chemicals as weapons in Syria", said Ban.

The investigative team would also have "full access to all locations" and "may establish contact and receive information from any parties" in Syria.

The United States had pushed for the UN chemical weapons probe after a wave of chlorine gas attacks that the West blamed on President Bashar al-Assad's forces.

Ban did not specify in his letter exactly when the panel would begin its work, however, the team is to present its first findings to the council 90 days after it begins the investigation.

av/lw (AP, AFP)