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.
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.