The United Nations observer mission in Syria has formally ended, in line with Thursday’s Security Council decision.
The team was deployed to monitor a ceasefire between rebels and the government agreed as part of former UN envoy Kofi Annan’s six-point peace plan, but the truce never took hold.
The UN decided to end the mission in response to growing levels of violence.
Mr Annan’s successor, Lakhdar Brahimi, said on Sunday his task was no longer to prevent a civil war, but to end one.
“Civil war is the cruellest kind of conflict, when a neighbour kills his neighbour and sometimes his brother. It is the worst of conflicts,” the newly appointed Mr Brahimi told France 24 television.
“There are a lot of people who say that we must avoid civil war in Syria, but I believe that there has already been a civil war there for some time now. What’s necessary is to stop the civil war and that is not going to be easy.”
Mr Annan quit at the beginning of August, saying the increasing militarisation of the conflict as well as a lack of unity in the UN made it impossible for him to carry out his task.
Russia and China have vetoed resolutions on the crisis three times, citing their opposition to any action which might be seen as regime change imposed from outside.
The appointment of Mr Brahimi, 78, an Algerian who has held a long list of high-profile diplomatic posts, was widely welcomed by the international community on Sunday.
Officials in Damascus have also offered their support.
Analysts say he has a formidable reputation at the UN but is also seen as independent of the major powers.
However, opposition groups have expressed scepticism about his ability to accomplish his mission.
On Sunday, a dispute flared between Mr Brahimi and the Turkey-based opposition Syrian National Council after he said it was too soon for him to comment on whether President Bashar al-Assad should step down.
The group said Mr Brahimi’s comments were “unacceptable” and called for him to retract them and apologise.
Mr Brahimi later told Al-Jazeera TV that the SNC had misinterpreted his comments, and in turn demanded that the group apologise to him.