Aid organisations stepped up calls for civilians trapped in the Syrian town of Qusayr to be evacuated.
Powerful explosions struck the outskirts of Damascus early on Sunday, sending columns of fire into the night sky.
A car bomb has killed 53 people and wounded 200 in central Damascus when it blew up on a busy highway close to ruling Baath Party offices.
A car bomb shook the centre of Damascus, killing several people and incinerating cars on a busy highway close to offices of the ruling Baath party.
The head of Syria's main opposition coalition, Mouaz Alkhatib, has said he was ready to hold talks with representatives of President Bashar al-Assad.
Syrian President Bashar al-Assad will deliver a rare speech on Sunday about the uprising against his rule, which has killed 60000 people.
Nato has told President Bashar al-Assad that any use of chemical weapons in his fight against rebel forces would be met by an international response.
Fighting has erupted in a Damascus suburb and around a Syrian army base as a truce marking a Muslim holiday crumbled almost before it had begun.
UN observers say 13 bodies have been discovered bound and shot in eastern Syria, days after a massacre of 108 civilians ignited a world outcry.
Syria has challenged the UN chief over the scope of a UN truce monitoring mission, resisting a larger presence as its army shelled targets in Homs.