The United Nations says there is clear evidence that rockets carried sarin gas into a Damascus suburb and that there has been wide-scale use of chemical arms in the Syria war.
UN leader Ban Ki-moon slammed the use of banned poison arms as a war crime and called on the UN Security Council to impose "consequences" to back a Russia-US plan to eliminate President Bashar al-Assad's chemical arsenal.
Without naming the perpetrator, Ban said an August 21 attack on Damascus suburb of Ghouta was "the most significant" use of chemical weapons since late Iraqi dictator Saddam Hussein killed thousands in the town of Halabja in 1988.
The United States says more than 1400 people died in Ghouta. Its threat of a military strike has eased following the agreement of a plan with Russia.
But the UN experts' report released today will now become a key weapon in a Security Council battle over how much of threat must be made against Assad to make him disarm.
While Assad blames opposition rebels for the attack, France's Foreign Minister Laurent Fabius was quick to state that the UN report left "no doubt" that government forces were to blame.
"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" in Ghouta, said the report of UN inspectors who were in Syria when the attack was staged.
The experts said they had concluded that "chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic, also against civilians including children on a relatively large scale."
A UN-mandated independent commission of inquiry into rights violations in the Syria war announced separately on Monday that it was investigating 14 alleged chemical weapons attacks in Syria.
The UN experts went to Damascus on August 18 to investigate claims that chemical weapons were used at Khan al-Asal, near Aleppo on March 19 and at Sheik Maqsood and Saraqueb earlier.
The team was told to focus on the Ghouta attack as soon as the horrific events became known. They will return later to investigate the other sites.
Ban called on the Security Council to impose "consequences" for any failure by Assad to keep to a Russia-US plan to destroy Syria's banned chemical arsenal in the next year.
He told the 15-nation council the UN investigators have "now confirmed, unequivocally and objectively that chemical weapons have been used in Syria."
"This is a war crime," he added.
Ban told how doctors treated civilians with no external signs of injuries in the streets of Ghouta in the hours after the attack.
France and Britain will soon send a draft resolution to other Security Council members demanding a threat of sanctions if Assad does not keep to a disarmament plan and for the chemical attacks to be referred to the International Criminal Court, diplomats said.
The council is expected to start negotiations this week on a resolution.