Mediator Lakhdar Brahimi will meet representatives of the warring sides in Syria separately for the first few days to increase pressure on them.
The first round of peace talks has ended with no progess made on ending the civil war or commitment from the govt to return to the negotiating table.
Syria's opposition coalition has said it will not engage in international peace talks while Hezbollah backs President Bashar al-Assad's forces.
Syria's opposition resumed talks aimed at closing their fractious ranks, as government forces launched a fierce onslaught on a rebel-held border town.
Syrian opposition leader met Russia's foreign minister, opening a window to a possible breakthrough in efforts to broker an end to Syria's civil war.
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.
Continued military bombardments in central Syria is dashing the prospects of a UN-brokered ceasefire taking hold.