Fighting between Syrian troops and rebels in the capital Damascus reportedly continues ahead of a meeting between Russian president Vladimir Putin and peace envoy Kofi Annan.
Activists claim forces loyal to Syria's President Bashar al Assad have been using helicopter gunships in the clashes, described as the city's worst since the uprising started 16 months ago.
The Britain-based Syrian Observatory for Human Rights and Damascus-based activist Maath al Shami said the attacks were focused on the neighborhoods of Qadam and Hajar al Aswad, while fighting also took place in Kfar Souseh, Nahr Aisha, Midan and Qadam.
Syria's state-run news agency said troops were still chasing "terrorist elements" who fled from Nahr Aisha to Midan.
But the revolutionary Free Syrian Army said the operation - dubbed "the Damascus volcano and earthquakes of Syria" - was launched "in response to massacres and barbaric crimes" committed by the Assad regime.
The FSA said in a statement that it started to conduct "attacks on all security stations and branches in the cities and the countryside, to enter into fierce clashes (with their forces) and to call on them to surrender".
It called for all international roads to be cut off, "from (northern) Aleppo to (southern) Daraa and from (eastern) Deir Ezzor to (coastal) Latakia, to cut off and seize the supply lines."
Meanwhile, as Mr Putin prepared to hold talks with Mr Annan, Russia slammed as "blackmail" Western pressure to push for a UN Security Council resolution against Syria's regime.
Russian Foreign Minister Sergey Lavrov, speaking after his own discussions with Mr Annan, said: "To our great regret, there are elements of blackmail. We are being told that if you do not agree to passing the resolution under Chapter 7 of the UN Charter, then we shall refuse to extend the mandate of the monitoring mission."
Fears were also raised by comments from Nawaf Fares, the first Syrian ambassador to defect, who warned that President Assad would use chemical weapons against opposition forces and may have already deployed them.
Mr Fares, the most prominent politician to defect since the uprising against Assad began, insisted the president's days were numbered but warned he would be prepared "to eradicate the entire Syrian people" to remain in power.
His comments came as a Turkish official said a Syrian brigadier-general and several other defected military officers were among 1,280 Syrians to have fled from Syria to Turkey overnight.
