Ban spoke shortly before the chief chemical weapons inspector, Ake Sellstrom, told The Associated Press that he would deliver his report to the secretary-general in New York this weekend.
Ban also said President Bashar Assad's regime "has committed many crimes against humanity."
"Therefore, I'm sure that there will be surely the process of accountability when everything is over," he said.
Asked whether Ban's conclusion was in response to the report, U.N. associate spokesman Farhan Haq said that as far as he knows the report hadn't been completed "so it's not possible for any of us to have seen the report at this present moment."
But he added that Ban "has been in touch with different people including the experts."
The secretary-general spoke at the Women's International Forum. He thought his speech and response to questions were not being broadcast, but they were shown on U.N. television.
Speaking by telephone from the Netherlands, Sellstrom said he didn't know exactly when the report would be released publicly. He said that "it's done, but when to present it is up to the secretary-general." But in a later conversation Friday, Sellstrom said he wasn't quite finished with the report, and that what he meant was that it would be done once he delivered it to Ban this weekend.
The two U.N. diplomats said the inspectors had soil, blood and urine samples and may also have collected remnants of the rockets or other weapons used in the attack, which could point to those responsible. The diplomats spoke on condition of anonymity because discussions on the issue have been confidential.
Haq said after receiving the report, Ban will present it U.N. member states and that the media should receive it shortly afterward.
In his speech, Ban said "the disaster" in Syria has created "a lost generation of children and young people" and led to "rising sectarian tensions, regional instability, the largest displacements of people in a generation, grave violations of human rights, including sexual violence."
"The latest fighting has also raised the specter of chemical warfare which, if confirmed by the U.N. investigation mission, would be an atrocious violation of international law," Ban said.