A US and Russian-inked programme to destroy Syria's chemical weapons should begin "in a matter of days," the watchdog tasked with implementing the plan said Monday.

The Organisation for the Prohibition of Chemical Weapons (OPCW) said that its experts "are already at work preparing a roadmap that anticipates the various challenges involved in verifying Syria's declared stockpiles."

"It is envisaged... that the programme to eliminate chemical weapons in Syria will be initiated in a matter of days," the Hague-based OPCW said.

The plan was thrashed out by Russia and the United States in Geneva at the weekend to avert US-led military strikes on Syria, which is accused by Washington of having killed hundreds of people in a gas attack near Damascus on August 21.

President Bashar al-Assad's regime is to provide "on an expeditious basis" a complete inventory of its chemical weapons, production facilities, and "related materials" to the OPCW.

Watchdog experts "will verify the accuracy of this disclosure with on-site inspections, and will also assist in putting into place arrangements to keep the warfare materials and the relevant facilities secure until their destruction."

UN Secretary General Ban Ki-moon on Monday called the use of chemical weapons in Syria a war crime after UN investigators published their report saying they had indisputable evidence of their use in the August attack.

The United States, which has threatened a military strike over the chemical arms, estimates that 1,400 people died in the attack.

Ban called on the Security Council to impose "consequences" for any failure by President Bashar al-Assad to keep to the Russia-US plan to destroy Syria's banned chemical arsenal.

Under the deal, which aims to destroy Syria's chemical arsenal by mid-2014, Syria joins the Chemical Weapons Convention, which will come into force in Syria on October 14, the OPCW said.

The 41-member OPCW Executive Council is expected to meet on Thursday or Friday.

"I am aware of the onerous responsibility that the international community is placing on our shoulders," OPCW Director General Ahmet Uzumcu was quoted as saying.

"We will bring to bear on this mission our full energies and commitment, and I have every confidence that the international community will support us fully."

Though the military threat against Syria has eased, Monday's UN report will influence what measures are taken by the UN and the OPCW to make Assad stick to the plan.

Washington and Paris are pushing for military action but are facing deep public scepticism, despite the deaths of more than 110,000 people since an uprising against Syria's regime began in March 2011.