The five permanent members of the UN security council reached an agreement on Thursday over the wording of a "binding and enforceable" resolution to eliminate Syria's stockpiles of chemical weapons.
British and US officials announced the breakthrough after a fast-moving day of diplomacy on the margins of the United Nations general assembly in New York.
But the agreement does not authorise the use of force if Syria does not comply – the sticking point that had prevented diplomatic progress on the conflict that has lasted more than two years and killed more than 100,000 people.
The British ambassador to the UN, Mark Lyall Grant, said in a post on Twitter that the five permanent members of the security council – Britain, France, the US, Russia and China – agreed on a "binding and enforceable draft" of a resolution. He said the text would be introduced to the 10 other members of the security council at a meeting later.
The development was announced after hastily convened talks between the US secretary of state, John Kerry, and his Russian counterpart, Sergei Lavrov. If the resolution is adopted, it would be the first legally binding resolution on the Syrian conflict.
US officials said the deal was significant. The administration, in a statement attributed to a state department official, said it was "historic and unprecedented".
The statement said: "This is a breakthrough arrived at through hard-fought diplomacy. Just two weeks ago, no one thought this was in the vicinity of possible."
However, in order to get the agreement, the US had to concede that the wording of the resolution would not fall under chapter 7 of the UN charter, which allow it to be enforced by military action.