The UK is to put a resolution to the UN Security Council later on Wednesday “authorising necessary measures to protect civilians” in Syria.
The resolution will be put forward at a meeting of the five permanent members of the council, UK Prime Minister David Cameron said on Twitter.
Earlier a team of UN weapons inspectors resumed work probing an alleged chemical weapons attack on 21 August.
UN Secretary General Ban Ki Moon appealed on the council to take action.
“The body interested with maintaining international peace and security cannot be ‘missing in action’,” Mr Ban said.
“The council must at last find the unity to act. It must use its authority for peace,” he went on.