The investigation could be broader than the Syrian government's request for an independent probe of a purported chemical weapons attack on Tuesday. Ban said he was aware of other, similar allegations and hoped the probe would help secure Syria's chemical weapons stockpile.
Syria is widely believed to have a large stockpile of chemical weapons. The government has not confirmed it, saying only that it would never use chemical weapons against its own people.
"My announcement should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity," the secretary-general said. "The international community needs full assurance that chemical weapons stockpiles are verifiably safeguarded."
Western nations fear President Bashar Assad would use chemical weapons if he sees the two-year war turning against his government. But they are equally concerned that rebel forces, including some linked to al-Qaida, could get their hands on unguarded chemical weapons or the materials to make them.
Ban said investigators would look into Syria's allegation that rebels carried out a chemical weapons attack on Khan al-Assal village in northern Aleppo province.
The rebels denied the attack and blamed regime forces. The Syrian National Coalition, the main opposition group, also demanded an international investigation.
The secretary-general said he was aware of "other allegations of similar cases involving the reported use of chemical weapons," but did not make clear whether these would be part of the investigation.
France's U.N. Ambassador Gerard Araud said Wednesday that the Syrian National Coalition had reported a second chemical weapons attack Tuesday in the Damascus area. France and Britain said they would ask Ban to have the U.N. investigate both incidents.
He said his senior advisers are working to set up an investigation in close consultation with the Organization for the Prohibition of Chemical Weapons which oversees the chemical weapons convention, and the World Health Organization. He said issues to be decided include the overall mandate, the composition, and operational conditions, including safety and security.
The investigation will start "as soon as practically possible," Ban said, but "will not happen overnight."
He said full cooperation from all parties will be essential and stressed that this includes "unfettered access."
As the situation in Syria worsens, Ban said, "the international community's concern about the safety and security of chemical weapons stockpiles as well as possible use by all parties has increased."
Ban said he has spoken out repeatedly on the Syrian government's primary responsibility to ensure the safety and security of any chemical weapons and sent two letters to President Bashar Assad "to remind him of this solemn duty."
"It is my hope that the mission would contribute to ensuring the safety and security of chemical weapons stockpiles in Syria," he said.
With more than 70,000 people killed and no end to the violence in sight, the secretary-general reiterated that "the military solution in Syria is leading to the dissolution of Syria."
He called on the deeply divided region and international community to find unity and support efforts by the joint U.N.-Arab League envoy Lakhdar Brahimi to help the Syrian people reach a political solution and end the conflict.