Russia and other big powers have agreed to back a proposal by UN envoy Kofi Annan for a national unity government to lead political change in Syria.
Western diplomats said the proposed cabinet could include members of the opposition and government, but no-one who would undermine its credibility.
The proposal will be discussed at a meeting of the UN Action Group on Syria on Saturday.
President Bashar al-Assad has said Syria is in a "real state of war".
According to an unnamed diplomat quoted by Reuters, Mr Annan made clear that any settlement should also be irreversible, with clear transition steps and a fixed timeline.
"These include establishing a transitional national unity government to create a neutral backdrop for transition," the diplomat said.
"It could comprise present government members, opposition and others, but would need to exclude those whose continued participation or presence would jeopardise the transition's credibility, or harm prospects for reconciliation and stability."