Fierce clashes broke out near the Syrian capital on Sunday as President Bashar al-Assad's opponents sought to crank up the pressure for UN action after the Arab League withdrew its observers.
The day's toll of at least 80 people killed across Syria, half of them civilians, raised the count since Friday alone to 175 dead, activists from the Syrian Observatory for Human Rights said.
The Observatory's head, Rami Abdel Rahman, told AFP that the clashes near Damascus were the fiercest since the anti-regime revolt broke out in mid-March.
Regime forces fired heavy artillery and mortar rounds against the Damascus suburbs of Douma, Saaba, Irbin and Hamuriyeh and were locked in close combat with rebel fighters emboldened by a fresh wave of desertions, activists said.
The rebel Free Syrian Army said 50 more officers and soldiers turned their back on Assad and in a "steady progression of fighting towards the capital" clashed with army regulars only eight kilometres (five miles) from Damascus.
The regime, in turn, has launched "an unprecedented offensive in the past 24 hours, using heavy artillery" against villages in Damascus and Hama province of central Syria, the rebel army said.
It reported clashes as close as four kilometres to the capital.
"The more the regime uses the army, the more soldiers defect," Ahmed al-Khatib, a local rebel council member on the Damascus outskirts, told AFP.
Other rebel sources reported heavy fighting in Rankus, 45 kilometres from Damascus, and of heightened tension in Hama, further to the north.
Rankus was "besieged for the past five days and is being randomly shelled since dawn by tanks and artillery rounds," rebel Abu Ali al-Rankusi told AFP by telephone.
In Hama, pro-regime snipers were deployed on the rooftops, according to activists, with security forces leaving "bodies of dead people with their hands tied behind their backs" on the streets across several neighbourhoods.
In addition to 40 civilians, the London-based rights group said, 26 soldiers, five other members of the security forces and nine army deserters were also among those killed on Sunday.
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The watchdog said the regime soldiers were killed in three separate attacks in the Idlib region of northwest Syria and near Damascus, while the official media reported 16 soldiers killed.
The latest spike in violence, on top of what the United Nations said at the start of January already added up to 5,400 killings, pushed the Arab League to suspend its mission to Syria in a surprise move on Saturday.
Arab foreign ministers are to meet in Cairo on February 5 to review the suspension, a League official said.
League chief Nabil al-Arabi, departing Cairo for the United Nations, said the decision was taken after Damascus "chose the option of escalation," but Moscow condemned at the move.
"We would like to know why they are treating such a useful instrument in this way," Foreign Minister Sergei Lavrov said.
The 165 observers deployed a month ago after Damascus agreed to an Arab League plan foreseeing a halt to the violence, prisoners freed, tanks withdrawn from built-up areas and free movement of observers and foreign media.
Arabi said on Sunday he hopes Moscow and Beijing will allow the UN Security Council to issue a resolution backing a new League plan to end the crisis.
"I hope these two countries will alter their position concerning the draft UN Security Council resolution which would adopt the Arab plan," he said, according to Egypt's official MENA news agency.
This plan looks to a halt in the violence and Assad transferring power to his deputy ahead of negotiations -- a formula flatly rejected by Damascus.
Moscow opposes the draft UN resolution, and it has proposed its own draft assigning equal blame for the violence on both Assad and the opposition, an option dismissed by the West.
Russia has close trade ties with its Soviet-era ally, signing a new warplane delivery contract with Damascus this month, and it leases a Syrian port on the Mediterranean for its navy.
UN chief Ban Ki-moon said Sunday that Assad must end the killings.
"First and foremost, he must stop immediately the bloodshed," Ban told reporters. "The Syrian leadership should take a decisive action at this time to stop this violence. All the violence must stop."