Ban will present the report to the UN Security Council. He has already revealed that he expects the report by a UN investigation team to give "overwhelming" confirmation that arms were used in an attack near Damascus on August 21 in which hundreds died.
But the UN team is not allowed to say who carried out the attack, which the West blames on Assad. While diplomats say the details will give a clear pointer to who is responsible, opponents and supporters of Assad, who pleads innocence, will be looking for evidence to back their case.
A Russia-US accord on the dismantling of Syria's chemical stockpile will also weigh heavily on Security Council consultations expected to be called on Monday.
Secretary General Ban Ki-moon to present a report on chemical weapons in Syria
Russia's Foreign Minister Sergei Lavrov on Saturday slammed what he called attempts to "retouch" the UN report. Syria's UN envoy, Bashar Jaafari, has also said his government will not accept a "politicised" report.
"Russia, the Americans, all sides, have been putting on pressure over this report," a UN official told, speaking on condition of anonymity. "Ban's office has agonised over every word. The message has to be how serious this attack was but also support the Russia-US initiative."
The head of the UN team, Swedish expert Ake Sellstrom, arrived in New York on Sunday and met Ban, officials said.