Syrian rebels killed at least 25 members of a mainly Alawite militia loyal to President Bashar al-Assad on Friday and troops killed 10 anti-Assad protesters in the northern city of Aleppo, opposition activists said.
The violence, which followed a day of bloodshed across Syria in which 125 deaths were reported, is further evidence of a slide towards sectarian-tinged civil war in a country where majority Sunni Muslims are spearheading an anti-Assad revolt.
Russia, one of Assad's strongest backers abroad, said the Syrian authorities were ready to withdraw troops from cities, a main plank of envoy Kofi Annan's never-implemented peace plan, "simultaneously" with rebels.
Russian Foreign Minister Sergei Lavrov said after talks with his Syrian counterpart that he had urged Syria to "do a lot more" to implement Annan's UN-backed proposals, but that foreign countries must also press rebels to stop the violence.
Hundreds of people have been killed in Syria since the UN-Arab League mediator announced his six-point plan on April 12 and there was no let-up in the violence on Friday.
"Armed terrorist groups committed a brutal massacre against 25 citizens in Darat Azzah," state TV reported, saying more were missing from the village in the northern province of Aleppo.
Several men covered in blood and piled on top of each other on a roadside, some in army fatigues and some in t-shirts, could be seen in a video link sent by the Syrian Observatory for Human Rights, apparently showing the aftermath of the same incident.
The British-based opposition watchdog said 26 men believed to be pro-Assad "Shabbiha" militiamen had been killed.
Activists in the north said the area had been the scene of heavy army shelling on Sunni villages, as well as rebel attacks on troops and Shabbiha deployed on the main roads.
Assad's foes accuse security forces and Shabbiha militiamen of perpetrating many abuses against civilians, including mass killings, in the uprising that began in March last year with peaceful protests against four decades of Assad family rule.
In Aleppo, Syria's business hub, thousands of demonstrators were marching toward the central Saadallah al-Jabiri Square when four armoured vehicles fired on them, activists told Reuters by telephone. Two of them said they were speaking from Aleppo.
"The wounded were taken to houses and are trapped there. They cannot be transported to hospitals because troops and Shabbiha are surrounding the neighbourhood," one of them said.
Aleppo, along with central Damascus, had stayed relatively quiet in the early months of the revolt that engulfed many other provincial cities, but unrest has gradually spread there too.
Activist video footage showed a large crowd of protesters, some draped in revolutionary flags, running along a street as heavy gunfire cracked out. Another video showed a man whose chest was covered in blood being dragged along the road.
The Syrian Observatory for Human Rights reported heavy government shelling on opposition strongholds in Idlib, Deraa and Homs provinces, as well as fighting between troops and rebels in the eastern city of Deir al-Zor on Friday, a day when anti-Assad protests often erupt after Muslim prayers.
The 46-year-old leader's power rests mainly on the military and a cluster of security agencies dominated by his minority Alawite sect, a distant offshoot of Shi'ite Islam.
Four brothers, two brigadier-generals and two colonels, announced their defection from the army in a video posted on the Internet on Friday, a day after a Syrian air force pilot flew his MiG-21 fighter plane to neighbouring Jordan.
The brothers come from the northern province of Idlib, but do not appear to have had frontline roles in months of fighting there. Two worked as doctors in the Aleppo military hospital, one was an inspector and one was an air force instructor.
The armed forces have suffered a trickle of defections to the opposition, but have remained mostly loyal despite the strain of battling an increasingly potent insurgency.
The violence has driven many Syrians from their homes and 1.5 million now need humanitarian help, up from a million in March, but aid agencies are struggling to reach them.
They include 350,000 in Idlib province and some 250,000 in the flashpoint city of Homs, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.
"More and more people are leaving their homes, vulnerability is growing and assistance is needed," OCHA spokesman Jens Laerke told a news briefing in Geneva. "The overall problem is the lack of security and the lack of access."
Aid workers hoping to evacuate hundreds of trapped civilians and wounded people could not enter parts of Homs on Thursday due to shooting, the International Committee of the Red Cross said.
More than 86,000 Syrian refugees have now been registered in Iraq, Jordan, Lebanon and Turkey - a rise of nearly 20,000 since May 31, OCHA said, quoting the UN refugee agency UNHCR.
The outside world, paralysed by differences between major powers, has proved impotent to halt the carnage in Syria, where Assad has brushed off diplomatic pressure and sanctions.
In the latest Western gesture of distaste, Britain has barred the head of Syria's National Olympic Committee, General Mowaffak Joumaa, from attending next month's Games in London, the BBC said. Mowaffak is regarded as a close friend of Assad.
The British Home Office (interior ministry) and foreign office declined to say whether he had been denied a visa.