A UN report says sarin gas was used in a rocket attack in the Syrian capital, Damascus, last month, although it has not attributed blame.
"This is a war crime," Secretary-General Ban Ki-moon said.
US allegations that the government was responsible led to threats of military action and then a US-Russia deal for Syria to make safe its chemical arms.
World powers will now try to hammer out a UN Security Council resolution.
Earlier, UN investigators said they were probing 14 alleged chemical attacks in Syria since September 2011.
Meanwhile, Turkey said it had shot down a Syrian helicopter close to its border. Deputy Prime Minister Bulent Arinc said the aircraft was engaged by fighter jets after violating Turkish air space. It came down in Syria and the fate of the crew is unknown.
'Despicable crime'
Mr Ban briefed the Security Council on the report, and then addressed the media.
He said he was submitting the UN mission's report "with a heavy heart".
"The mission has concluded that chemical weapons were used on a relatively large scale in the Ghouta area of Damascus [on 21 August]... The attack resulted in numerous casualties, particularly among civilians."
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Analysis
image of Jeremy Bowen
Jeremy Bowen
BBC Middle East editor, Damascus
I have heard some grumbling about the chemical weapons deal. Regime officials have said some are unhappy to give up what they viewed as potentially their most powerful weapon, and a deterrent of which Israel especially had to be wary.
But the key to whether President Bashar al-Assad will want to implement the agreement lies in his relationship with Moscow.
Assuming that Russia wants the Syrians to give up their chemical arsenal - and since it is giving up its own, there is no reason to doubt that - then President Assad will not want to alienate his most important friend.
The Russians sell Syria weapons and ammunition. Even more importantly, Russia has been watching Syria's back at the UN Security Council. That adds up to a lot of leverage.
Civil war far from over
Mr Ban spoke of the suffering of the victims.
"Survivors reported that following an attack with shelling, they quickly experienced a range of symptoms, including shortness of breath, disorientation, eye irritation, blurred vision, nausea, vomiting and general weakness.
"Many eventually lost consciousness. First responders described seeing a large number of individuals lying on the ground, many of them dead or unconscious."
The UN investigators examined blood, hair, urine and rocket samples.
Mr Ban said 85% of the blood samples tested positive for sarin.
He said: "On the basis of its analysis, the mission concluded that it - and I quote - 'collected clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used in the Ein Tarma, Moadamiyah and Zalmalka in the Ghouta area of Damascus.'"
The delivery vehicle was a variant of the M14 artillery rocket, fired from an unspecified region to the north-west.
Mr Ban added: "I trust all can join me in condemning this despicable crime. The international community has a responsibility to hold the perpetrators accountable."
He said the mission was unable to verify the number of casualties, but referred to the "terrible loss of life on 21 August".
Mr Ban added: "This is the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them in Halabja in 1988."
Assigning blame for the attack in Ghouta was not part of the inspectors' remit.
Laurent Fabius (C): "This resolution foresees what is going to happen if the Syrians do not respect their commitments"
Mr Ban was asked at a brief press conference whether he knew who was to behind the attack.
He said that "we may all have our own thoughts" but it was for "others to decide" what steps should be taken to bring those responsible to justice.
Mr Ban then said the matter would be the subject of "ongoing discussions in the Security Council", before adding: "I do not have a clear answer at this time."
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Syria conflict in numbers
Two million children have dropped out of school; 3,000 school buildings damaged or destroyed
Some 4.25 million people, mostly elderly and children, have abandoned their homes
Almost 200,000 living in overcrowded conditions with no clean water
Sources: UN, Reuters
However, US ambassador to the UN Samantha Power, citing the high quality of the sarin used and the type of rocket, said: "The technical details make it clear that only the regime could have carried out this large-scale attack."
Syrian President Bashar al-Assad has denied responsibility and blamed rebels.
Earlier, Paulo Pinheiro, the chairman of the UN Commission of Inquiry on Syria, said the commission had been investigating 14 alleged chemical attacks since it began monitoring Syrian human rights abuses in September 2011.
Mr Pinheiro said investigators had not so far been able to assign blame and were awaiting details from Monday's UN report.
He said the commission believed both President Assad's government and the rebels were responsible for war crimes, but that the regime alone had perpetrated crimes against humanity.
'Transparent and timely'
French President Francois Hollande and Mr Fabius earlier met British Foreign Secretary William Hague and US Secretary of State John Kerry in Paris on Monday to discuss the Syrian crisis.
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Syria's chemical weapons
CIA believes Syria's chemical weapons can be "delivered by aircraft, ballistic missile, and artillery rockets"
Syria believed to possess mustard gas and sarin, and also tried to develop more toxic nerve agents such as VX gas
Syria has not signed the Chemical Weapons Convention (CWC) or ratified the Biological and Toxin Weapons Convention (BTWC)
Sources: CSIS, RUSI
Western military options
Media review: Mixed response
The meeting follows the deal brokered at the weekend by Russia and the US under which Syria will disclose its chemical weapons within a week and eliminate them by mid-2014.
The US, UK and France said they were seeking a "strong" UN resolution with "serious consequences" if Syria failed to hand over its chemical arsenal, along with a "precise timetable" for dismantling it.
The UN Security Council is expected to draft a resolution in the coming days.
US President Barack Obama said on Monday: "If properly implemented, this agreement could end the threat these weapons pose not only to the Syrian people but to the rest of the world."
Mr Kerry said the UN resolution had to be "forceful, accountable, transparent and timely".
He said that all the countries involved, including Russia, had agreed that military intervention could be an option "should diplomacy fail".
"The framework fully commits the United States and Russia to impose measures under Chapter VII of the UN charter in the event of non-compliance."
Chapter VII permits military action if other measures do not succeed.
But Russian Foreign Minister Sergei Lavrov said any calls for swift UN action against Mr Assad showed a "lack of understanding" of the chemical weapons deal reached with Syria.
Mr Lavrov said: "Yes, our American colleagues would very much like there to be a Chapter VII resolution. But the final declaration, the final document that we approved and which has the guiding principles for how we proceed and for our mutual obligations, makes no mention of it."
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