UNITED NATIONS (AP) — Secretary-General Ban Ki-moon says the United Nations will investigate the possible use of chemical weapons in Syria.

He announced Thursday that the investigation would look into Syria’s allegation of an alleged chemical weapons attack in Aleppo, which Damascus blamed on rebels. The rebels have blamed the government.

Ban said he was aware of other allegations of the reported use of chemical weapons, but did not make clear whether these would be part of the investigation.

The secretary-general said he hopes the mission “will contribute to ensuring the safety and security of chemical weapons stockpiles in Syria.”

Ban said the investigation will start “as soon as practically possible” but “will not happen overnight.”

He said full cooperation from all sides will be essential and stressed that this includes “unfettered access.”