U.N. inspectors found "clear and convincing evidence" that sarin gas was used to attack rebel-held suburbs of the Syrian capital on Aug. 21 in violation of international law,
"This act is a war crime," Ban said in a statement to the U.N. Security Council accompanying the inspectors' report.
Hundreds of civilians, including children, died in the attacks that Western governments have blamed on the regime of Syrian President
Ban briefed the Security Council on the findings of an investigative team headed by Swedish biological and chemical arms expert Ake Sellstrom.
"The conclusion is that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic ... against civilians, including children, on a relatively large scale," the report said.
"In particular the environmental, chemical and medical samples we have collected provide clear and convincing evidence that surface to surface rockets containing the nerve agent sarin were used," it said, stating that the suspected breach of international law had been "unequivocally and objectively" confirmed.
The inspectors were not tasked with determining which side used the banned chemical agents. Assad has denied the accusations that his forces deployed the poison gas, claiming it was done by the rebels to draw Western support to their side.
Sellstrom delivered the report to Ban on Sunday, a day after U.S. Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov agreed on a plan for bringing Syria into compliance with the 1993 Chemical Weapons Convention and put its arsenal of poisons and nerve agents under international control.
While the U.N. investigators who reported to Ban focused their probe on the Aug. 21 attacks in Damascus suburbs, the head of the U.N. commission investigating war crimes disclosed in Geneva on Monday that 14 other suspected chemical attacks in Syria are being examined.
Commission Chairman Paulo Sergio Pinheiro said investigators have found evidence of war crimes committed by both Assad's forces and rebel groups, but that accusations of crimes against humanity are likely to be leveled only at the Damascus government because the rebels lack "a clear chain of command."
In Paris, Kerry wrapped up a weeklong diplomatic mission with a pledge alongside his British and French counterparts to push for a strong U.N. resolution to demand that Syria give up its chemical weapons.
"All options must remain on the table" to ensure that Assad complies with his pledge to surrender his chemical arms, Kerry said, referring to a U.S. threat to bomb Syrian government targets.
The diplomats said it was important to have U.N. backing for the plan outlined by the U.S. and Russia for Syria to turn over its entire chemical arsenal by the middle of next year. Officials estimate that the Syrian government possesses more than 1,000 tons of nerve agents and other deadly substances.
Although Britain, France and the United States are veto-wielding permanent members of the U.N. Security Council, where they will push their proposed resolution, China and Russia also have the power to scuttle action and are unlikely to back any measure imposing military sanctions on Syria. Russia is Assad's most influential ally and the regime's main arms supplier, and China opposes any action seen as outside interference in a sovereign nation's internal affairs.
The allied diplomats also reported that invitations were being issued to top Syrian opposition figures to meet in New York next week during the U.N. General Assembly session. Showing support for the opposition is a crucial lever for ensuring that Assad follows through on his pledge to give up his chemical arsenal, said French Foreign Minister Laurent Fabius.
"He must understand that there is no military victory, no possible military victory for him," Fabius said of Assad and his battle against those who seek to oust him.