The UK and 36 other countries have formally referred the latest allegations to Mr Ban, and called for inspectors "to be granted the necessary access to enable their investigation into these latest allegations as a matter of urgency".
Despite blocking a UN Security Council statement condemning the attack at an emergency meeting on Wednesday, Russia is supporting calls for an investigation.
The Kremlin suggested the attack could have been "premeditated provocation" by opposition forces in an attempt to win the backing of the UN.