Mediator Lakhdar Brahimi will meet representatives of the warring sides in Syria separately for the first few days to increase pressure on them.
State television has confirmed a helicopter crashed in Damascus but gave no details as Bashar al-Assad continues to bombard rebel-held districts.
More than 200 bodies have been found in basements and houses, and most were killed "execution-style" by troops on house-to-house raids.
Syrian Prime Minister Riyad Hijab is the latest to jump ship as defections continue to rattle Bashar al-Assad’s increasingly unstable regime.
Tanks have moved close to Aleppo but kept clear of new Turkish air defences installed to curb Syrian action near its frontiers.
Rebels are fighting security forces in Damascus in the most violent gun battles the Syrian capital has seen since the start of the year-long uprising.
Syria’s Al-Assad is arguably a war criminal, the US’s Hillary Clinton admitted, as the UN said his forces had killed more than 7 500 civilians.
Syrian forces shelled an opposition stronghold in Hama, killing 20 people, as two injured foreign journalists trapped in the city escaped to Lebanon.
Syrian troops have attacked rebel strongholds in Dera’a where the uprising against President Bashar al-Assad erupted nearly a year ago.
No casualties from Syria’s offensive on the city of Hama have been made available as communication was cut off in the residential neighbourhood.
Fighting has subsided in a rebel-held Syrian town after a fragile ceasefire took hold between President Bashar al-Assad’s forces and insurgents.
At least a dozen Syrian secret police have defected from an intelligence compound, activists say, in what appears to be the first major desertion.
Syrian troops shelled two northern villages overnight after an attack by army defectors on forces loyal to President Bashar al-Assad.
Over 50 people have been killed in Syria ahead of Friday prayers as a seven-month crackdown on pro-democracy protests becomes more violent.
The UN says that despite an agreement to end the assault on Syrian rebels, troops loyal to Bashar al-Assad have continued the brutal crackdown.
Bashar al-Assad’s opponents have taken a step toward unity by forming a national council to represent Syria’s uprising but have a long way to go.
President Bashar al-Assad’s opponents broke a taboo by appealing for foreign help to stop the killing of civilians.
An armoured Syrian force surrounded a town near the city of Homs on Monday and fired heavy machineguns after the defection of soldiers in the area.
Syrian forces killed at least three protesters as tens of thousands of people marched again to demand the removal of President Bashar al-Assad.
Syrian forces have killed dozens of protesters despite assurances by President Bashar al-Assad to Ban Ki-moon that a crackdown was over.
