Ban Ki-moon made no comment on who was to blame for the 21 August attack in the Ghouta area of Damascus, as that is not part of the report’s remit.
But he did say Syria’s president was guilty of “crimes against humanity”.
The US and Russian foreign ministers are continuing their talks on a plan to make safe Syria’s chemical weapons.
A spokeswoman for Russian Foreign Minister Sergei Lavrov said the talks with US Secretary of State John Kerry in Geneva would head into the night, adding: “They are working on some real substance.”
Washington and its allies accuse the Syrian government of killing hundreds of people in a chemical attack in Ghouta. The government denies the allegation, blaming rebels.
Mr Ban said the UN findings would be “an overwhelming report that chemical weapons were used”.
Mr Ban made no comment on blame but did say that Syrian President Bashar al-Assad had “carried out many crimes against humanity”.
“Therefore, I’m sure that there will be surely the process of accountability when everything is over,” Mr Ban said.
The BBC’s Nick Bryant says Mr Ban appeared not to have realised his comments at the UN Women’s International Forum were being broadcast.
But they were shown on UN television.
Chief chemical weapons inspector Ake Sellstrom has now confirmed that the report is complete.
“It’s done, but when to present it is up to the secretary-general,” he said.
US state department spokeswoman Marie Harf said the US believed the UN report would not assign blame but that it would “reinforce what we have already said” about the Ghouta incident.
Diplomats said the report might not lay explicit blame, but that its factual reporting – based on soil, blood and urine samples, and interviews with doctors and witnesses – could suggest who was responsible.
The negotiations in Geneva are mainly concerned with a Russian plan to place Syria’s chemical weapons under international control.
Syria has agreed to the plan and has sent documents to the UN to sign up to the Chemical Weapons Convention, which outlaws the production and use of the weapons.
The Russian foreign ministry spokeswoman said of the talks: “We are staying; probably they will finalise it through the night. I am not sure about tomorrow (Saturday), but they will go through the night.”
She told Reuters: “It is a sign that we are going on, that we proceed with talking and negotiating. Now it is like a real negotiating process, they are working on some real substance.”
Mr Kerry had earlier described the talks as “constructive” and President Obama said after meeting Kuwait’s emir at the White House that he hoped they would “bear fruit”.
“But I repeated what I’ve said publicly, which is any agreement needs to be verifiable and enforceable,” Mr Obama said.
Diplomacy will continue over the weekend, with Mr Kerry visiting Israel on Sunday.
UK Foreign Secretary William Hague and French counterpart Laurent Fabius will meet Mr Kerry in Paris on Monday.
Before resuming bilateral talks in Geneva, Mr Lavrov and Mr Kerry met UN-Arab League envoy Lakhdar Brahimi.
Mr Kerry said that they planned to meet again on the sidelines of the UN General Assembly in New York later in the month to try to set a date for a long-delayed peace conference known as Geneva 2.
He said both the US and Russia were “deeply committed to a negotiated solution” to the conflict in Syria and he and Mr Lavrov were “working hard to find the common ground to be able to make that happen”.
Mr Lavrov said he welcomed the chance to discuss the “longer-term goal” of peace in Syria, and that now Syria had joined the Chemical Weapons Convention it was necessary “to design a road which would make sure that this issue is resolved quickly, professionally, as soon as is practical”.
The BBC’s James Robbins in Geneva says it appears the pair of envoys get on well, but that until they agree on the narrower issue of chemical weapons, reviving the wider peace talks feels like a distant prospect.
If the Geneva talks are successful, the US hopes the disarmament process will be agreed in a UN Security Council resolution.
However, Russia regards as unacceptable any resolution backed by military force, or a resolution that blames the Syrian government for chemical attacks.
Moscow has already objected to a draft resolution that would be enforced by Chapter VII of the UN charter, which would in effect sanction the use of force if Syria failed in its obligations.
Russia, supported by China, has blocked three previous draft UN resolutions condemning the Assad government.
More than 100,000 people have died since the uprising against President Assad began in 2011. Millions of Syrians have been displaced.