The UN envoy to Syria, Lakhdar Brahimi, has said international law is clear that a collective UN Security Council decision is needed for military action to be carried out.
Speaking in Geneva, Mr Brahimi also said evidence suggests some chemical "substance" was used near Damascus, claiming the lives of hundreds.
His comments came as the German foreign minister urged all members of the council, especially Russia, to back Britain's draft resolution on Syria.
The prospect of a US and UK military intervention in Syria appears increasingly likely, but what would an attack by western forces involve?
A textile factory in Damascus has started producing gas masks in case of a repeat of a suspected chemical attack on the area.
Western countries including the UK are considering their response to Syria as evidence of a large-scale chemical attack in Damascus mounts.