Twenty Syrian soldiers were killed on Saturday and 53 wounded in clashes with presumed army deserters, while 10 security agents and a deserter were killed in a bus ambush, activists said.
"Twenty soldiers perished on Saturday and 53 were wounded in clashes between the regular army and presumed deserters in the Baba Amro district of Homs," said a statement from the Syrian Observatory for Human Rights received in Nicosia.
The violence, in which loyalist and anti-regime casualties predominated in what is the worst surge of killing in six months, came as 12 civilians died and several were wounded by government gunfire.
In the area of Homs, at least 12 civilians died from fire by snipers and machineguns, while an undetermined number of others were killed in their homes by security forces, the Observatory said.
UN chief Ban Ki-moon urged Syria "immediately" to end attacks on civilians following the deaths of dozens of people in a fierce crackdown on dissent by Syria's security forces on Friday, which was also condemned by the Arab League.
The Observatory said the bus was transporting security agents between the villages of Al-Habit and Kafrnabuda in Idlib province, close to the Turkish border, when it was ambushed "by armed men, probably deserters."
The clash left 10 security agents and a deserter dead, said the Britain-based watchdog, which earlier reported 17 soldiers killed late on Friday in the central city of Homs when presumed deserters attacked two checkpoints.
Homs and Hama provinces have been at the forefront of the anti-government protests that have been brutally put down by the security forces at a cost, according to the UN, of more than 3,000 lives, mostly civilians.
Ban's appeal came after activists said 36 people were killed on Friday by security forces during mass protests calling for the imposition of a Libya-style no-fly zone on Syria.
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Ban "appeals for military operations against civilians to stop at once," said his spokesman Martin Nesirky.
"The violence is unacceptable and must stop immediately," he added. "The calls of the Syrian people for change must be answered with far-reaching reforms, not repression and violence."
Friday's violence prompted fresh condemnation from the foreign ministers of the 22-strong Arab League, which has been trying to broker an end to the unrest that has rocked Syria since anti-regime protests erupted in March.
"The Arab ministerial committee expressed its rejection of the continued killings of civilians in Syria and expressed its hope that the Syrian government will take the necessary measures to protect them," they said in a statement.
An Arab League task force met Syrian President Bashar al-Assad on Wednesday in Damascus and is due to hold talks Sunday in Qatar with top Syrian officials to try to reach "serious results and an exit to the Syrian crisis," it said.
The Syrian foreign ministry accused the Arab committee of stoking dissent and said that Foreign Minister Walid Muallem will "inform the committee tomorrow of the true situation in Syria," the state-run news agency SANA reported.
The Arab task force is being influenced by "lies spread by television channels" and should have "helped to calm (the situation) and reach a solution to ensure the security and stability of Syria instead of reviving dissent."
A defecting army officer who has taken refuge in Turkey, Colonel Riad al-Asaad, claimed in July to have established an opposition armed force called the "Syrian Free Army" comprising military defectors but its strength and numbers are unknown.
The latest violence was the deadliest in nearly six months to occur on a Friday, the day worshippers emerging from weekly prayers at mosques defy the security forces and swarm the streets to rally against the regime.
The bloodiest Friday was on April 22, when the death toll reached 72.