THE United Nations is set to publish an "overwhelming report" that chemical weapons were used to gas hundreds of civilians in Syria last month, secretary-general Ban Ki-moon has said.

Speaking ahead of the publication of the report by UN inspectors, Mr Ban said that President Bashar Assad's regime "has committed many crimes against humanity".

He said the report would lay out the evidence that chemical weapons were deployed to kill hundreds of people on the outskirts of Damascus on August 21, but was unlikely to provide a definitive answer on who was responsible.

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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.

Mr 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".

He added: "The latest fighting has also raised the spectre of chemical warfare, which, if confirmed by the UN investigation mission, would be an atrocious violation of international law."

It came as US Secretary of State John Kerry said he had held "constructive" talks with his Russian counterpart Sergey Lavrov aimed at securing placing Syria's chemical weapons arsenal under international control.

As US and Russian chemical weapons experts met in a Geneva hotel yesterday to haggle over technical details that must be ironed out to reach a deal, Mr Kerry and Mr Lavrov met with UN-Arab League envoy Lakhdar Brahimi to examine political developments.

At the UN in Geneva, Mr Kerry, flanked by Mr Lavrov and Mr Brahimi, told reporters after an hour-long meeting on the second day of their talks that the chances for a second peace conference "will obviously depend on the capacity to have success here ... on the subject of the chemical weapons."

Mr Brahimi also met privately with Mr Kerry at a Geneva hotel yesterday to explore ways to resume international negotiations last held in Geneva in June 2012 aimed at ending the Syrian civil war.

Mr Lavrov said it was "very unfortunate that for a long time the Geneva communique was basically abandoned."

Mr Kerry and Mr Lavrov will meet again in New York toward the end of this month to try to fix a date for a second conference.

The talks were the latest event in a rapidly moving series, following the August 21 gas attack on suburbs in Damascus.

The US blames President Assad for the use of chemical weapons, although he denies that his government was involved and points to the rebels engaged in a two-year civil war against his government.

President Barack Obama had begun building a case for support at home and abroad for a punitive military strike on president Assad's forces, then changed tack and asked Congress for explicit authority for a limited strike.

With the prospect that he might lose a vote on action, president Obama said he would consider a Russian proposal calling for international control of president Assad's chemical weapons and their eventual destruction.

President Obama dispatched Mr Kerry to Geneva to hammer out the details of the proposal.

However, there was immediate disagreement on Thursday when the US rejected comments President Assad made on Russian TV, saying he would not provide details on his country's chemical arsenal until 30 days after it signs an international convention banning such weapons.