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.”
The BBC’s Barbara Plett, in New York, says that the implication is that Mr Assad would not form part of the government, although this has not been made explicit.
It is not yet clear either, she adds, whether Russia shares the Western interpretation that Mr Annan’s plan would exclude the Syrian president.
In April, following months of bloodshed, the Syrian government agreed to a six-point peace plan.
UN monitors were deployed to oversee a ceasefire but the truce never took hold and the monitors have suspended patrols.
US Secretary of State Hillary Clinton, who will attend Saturday’s meeting, said that if all the parties agreed to Mr Annan’s roadmap for political transition then there was “great hope that this perhaps can be a turning point in the very tragic circumstances affecting the Syrian people”.
The proposal, which will be discussed in Geneva between five permanent members of the Security Council as well as regional powers Turkey, Iraq, Kuwait and Qatar, comes as violence continues in Syria.
On Wednesday, at least seven people were killed during an attack on a pro-regime television station south of Damascus.
The headquarters of al-Ikhbariya TV, some 20km (13 miles) south of the capital, were destroyed in the attack. Syria’s Information Minister Omran al-Zoebi said some of the victims had been abducted, bound, and killed in cold blood.
The Ikhbariya attack followed fierce clashes in suburbs of the capital, Damascus, described by opposition activists as the worst there so far.
The UK-based Syrian Observatory for Human Rights said fighting had taken place near positions of the Republican Guard, which is led by President Assad’s younger brother Maher and has the role of protecting the capital.
The Observatory also reported violence on Wednesday in the central city of Homs, Deir al-Zour in the east and in Idlib in the north.
Mr Annan’s deputy envoy, Jean-Marie Guehenno, warned the UN Human Rights Council in Geneva on Wednesday that “time was running out”.
He was speaking shortly before a commission of inquiry gave details of its report on the one of the worst attacks on civilians since the conflict began – the 25 May Houla massacre in which 108 people died.
Commission chairman Paulo Pinheiro told the council that “forces loyal to the government may have been responsible for many of the deaths” but he said his team had been unable to determine who was behind the massacre.
Mr Pinheiro said the perpetrators were from one of three groups: “shabiha” or other local militia from neighbouring villages, perhaps acting with the army; anti-government armoured groups; or foreign groups.
“While the commission could not rule out the possibility of anti-government fighters being responsible for the killing, this was considered very much unlikely,” he said.
Syrian ambassador Faisal Khabbaz Hamoui condemned the meeting as “flagrantly political” and walked out of the hall.
Senior US intelligence officials have described the conflict between the Free Syrian Army (FSA) and the government as a “seesaw battle”, suggesting that it is likely to be a long, drawn-out struggle.
“The regime inner circle and those at the next level still seem to be holding fairly firm in support of the regime and Assad,” one official told Reuters.
The BBC’s Ian Pannell, who has spent the past two weeks with rebel groups in Idlib province, says that over the past two months there have been marked changes, with the rebels clearly getting weapons across the border and from the Syrian military.
The rebels are becoming more organised and are going on the offensive, he says, and are controlling large swathes of northern areas.
The UN says at least 10,000 people have been killed since pro-democracy protests began in March 2011. In June, the Syrian government reported that 6,947 Syrians had died, including at least 3,211 civilians and 2,566 security forces personnel.