Success of a new international inquiry aimed at assigning blame for chemical weapons attacks during Syria's four-year-old civil war will require full cooperation of all warring parties, U.N. Secretary-General Ban Ki-moon said on Thursday.

The remarks were included in a letter to the U.N. Security Council outlining his plans for an investigation into alleged gas attacks, to be conducted by the United Nations and Organization for the Prohibition of Chemical Weapons (OPCW).

"Success will ... depend on the full cooperation from all parties, including the Government of the Syrian Arab Republic and other parties in Syria," Ban said in the letter, seen by Reuters.

The aim is "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, including chlorine or any other toxic chemical."

Ban said the inquiry will be chaired by an assistant secretary-general and two deputies. Earlier on Thursday he said in a statement that "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."

He did not say who will chair the inquiry.

Ban's letter is in response to the Aug. 7 adoption of a council resolution calling for an investigation.

The council is expected to respond to Ban within five days.

The investigation will have a one-year, renewable mandate.

Syrian government and opposition forces have denied using chemical weapons. Western powers say the government has been responsible for chemical attacks, including chlorine attacks. The Syrian government and Russia have accused rebel forces of using poison gas.

Ban was asked to set up the investigation after the United States struck a deal with Russia, a strong backer of Syrian President Bashar al-Assad, to demand an investigation.

Syria agreed to destroy its chemical weapons in 2013 in a bid to avoid U.S. military strikes threatened over a sarin gas attack that killed hundreds of civilians. The OPCW has since found chlorine has been "systematically and repeatedly" used as a weapon, though it is not mandated to lay blame.

A separate U.N. investigation had previously determined that sarin gas was used repeatedly in Syria to deadly effect, but that inquiry was also barred from assigning blame.

Chlorine's use as a weapon is prohibited under the 1997 Chemical Weapons Convention, which Syria joined in 2013. If inhaled, chlorine gas turns to hydrochloric acid in the lungs, which can be deadly.