Former peace envoy Kofi Annan has warned that the violent struggle between the Syrian government and opposition forces risks tearing the country apart.
He said it could spill over Syria's borders to spread sectarian war across the region.
Mr Annan called on the international community to "get its act together".
He said his successor Lakhdar Brahimi will need the support of all members of the UN Security Council - including Russia - to have a chance of a peaceful resolution to the Syrian crisis.
Mr Annan quit as the UN and Arab League's envoy to Syria in August, complaining of a lack of support from the Security Council.
Russia and China have blocked resolutions designed to put pressure on President Bashar al-Assad.
The former UN secretary general told the BBC Radio 4 Today programme: "If the international community doesn't get its act together and find a way of pressuring the parties to give up their ambitions on the battlefield, we are going to see a situation where sectarian war will spread in Syria and probably go beyond its borders.
"Syria is not Libya. Libya imploded, Syria will not implode but in all likelihood explode, and explode beyond its borders.
"That mosaic of Syria, if we tear it apart, is going to be very difficult to put together again."
Mr Annan warned that "there can be no military solution" to the challenge presented to Mr Assad's regime by the uprising which began in March 2011.
But he said: "Today it looks as if both sides have decided the way to resolve this is through the battlefield, which is in my judgement the wrong approach.
“It is going to lead to protracted war with the possibility of enflaming the region and killing thousands and thousands of Syrians."
Mr Annan said that veteran Algerian diplomat Mr Brahimi, who took up his post as peace envoy at the start of this month, will be dependent on strong support from the Security Council to make an impact.
"Lakhdar Brahimi is experienced, he can make a contribution if he gets the support he needs - not passive support, but real support from the Security Council," he said.
Asked if this must include Russia, Mr Annan replied: "Exactly. Alone, he cannot do it."
