The US and Russia have reached an agreement on a draft UN Security Council resolution aimed at ridding Syria of its chemical weapons.

The agreement ends diplomatic deadlock after a chemical weapons attack blamed on President Bashar al-Assad forces in Damascus on 21 August.

US ambassador to the United Nations Samantha Power said a deal was struck with Russia "legally obligating" Syria to give up its chemical stockpile and the measure would go to the full Security Council later.

Russian Foreign Minister Sergei Lavrov said an "understanding" had been hammered out.

The US had been negotiating on the sidelines of the UN General Assembly with Russia, President Assad's chief ally.

The aim was to craft a measure to demand the destruction of Syria's chemical arsenal in line with a US-Russian deal reached earlier this month that averted US military strikes on Syria in the midst of Syria's civil war.

Western powers on the Security Council backed away from many of their initial demands, diplomats say, in order to secure Russia's approval.

A major sticking point between Russia and Western powers was whether the resolution would be under Chapter 7 of the UN charter.

That covers the council's authority to enforce its decisions with measures such as sanctions or military force.

The compromise draft resolution makes the measure legally binding, but provides for no means of automatic enforcement with sanctions or military force.

Originally, the US, Britain and France had wanted the resolution to state explicitly that it was under Chapter 7.

The only reference to enforcement in the draft is a threat that if Syria fails to comply with the resolution, the council would impose punitive measures under Chapter 7, which would require a second resolution that Russia could veto.