The Syrian government accused rebels of carrying out a "brutal massacre" of 25 of its supporters on Friday in a flashpoint northwestern district that has been the scene of fierce clashes.
The independent Syrian Observatory for Human Rights gave a higher toll, saying that at least 26 government supporters -- most of them members of the feared shabiha militia -- had been killed.
The Britain-based watchdog said security forces also killed four civilian demonstrators in Syria's second-largest city Aleppo as opposition activists called nationwide protests on the Muslim day of prayer and rest.
Renewed bombardment by government troops of besieged rebel neighbourhoods of third city Homs, meanwhile, scuppered a new Red Cross attempt to evacuate trapped civilians as the United Nations said up to 1.5 million people needed aid.
"Armed terrorist groups... kidnapped a number of citizens in Daret Azzeh area in the countryside of Aleppo, according to official sources in the province," the state SANA news agency said.
"The terrorist groups... committed a brutal massacre against the citizens... through shooting them dead and then mutilating their bodies." it added.
"Initial information indicates that more than 25 of the kidnapped citizens were killed... with the fate of the rest of the kidnapped people still unknown." Amateur video posted on YouTube and distributed by the Observatory showed piles of mangled bodies of young men, their clothing soaked in blood. At least two of the bodies in the footage were wearing fatigues.
"These are shabiha of Bashar al-Assad's regime," the narrator said, without identifying himself.
In Aleppo -- Syria's commercial hub which had long been spared the violence triggered by the uprising which broke out in March last year -- troops opened fire on demonstrators killing four, the Observatory said.
Security forces also resorted to live rounds in a bid to disperse a protest in the upscale Mazzeh district of Damascus, the watchdog added.
In Homs, activists reached by AFP via Skype spoke of a "catastrophic situation" in the Old City and adjacent neighbourhoods after violence nationwide killed 168 people on Thursday, the highest single-day death toll since a UN-backed ceasefire was supposed to take effect on April 12.
"They have been shelling for months and today they continued," activist Abu Bilal said.
He said most Homs residents had fled and those who remain are trying to escape. "Seventy percent of the city's infrastructure has been destroyed," he said.
The International Committee of the Red Cross, which made two attempts to enter Homs on Thursday alongside the Syrian Red Crescent, said any new relief effort would depend on better security.
"We cannot know when our team will return to Homs, after they returned to Damascus yesterday following two failed attempts to evacuate civilians from the city," ICRC spokesperson Rabab al-Rifai told AFP.
"We will discuss the next steps internally, and in partnership with the Syrian Arab Red Crescent, before we take any decision regarding our return there."
The UN Office for the Coordination of Humanitarian Affairs (OCHA) said up to 1.5 million Syrians now need humanitarian aid, up from the one million estimated at the end of March.
"The humanitarian situation in Syria continues to deteriorate," said the latest OCHA bulletin.
"It is now estimated that up to 1.5 million people need humanitarian assistance."
In Homs province, 250,000 people need aid, OCHA said.
'Turkey does not ship weapons'
In Idlib province in the northwest, a hive of rebel activity near the Turkish border, the Red Crescent says that 350,000 people are needy.
Turkey denied reports it was shipping weapons to fighters of the Free Syrian Army.
"Turkey does not ship weapons to any neighbouring country, including Syria," foreign ministry spokesperson Selcuk Unal said.
The New York Times newspaper reported on Thursday that US intelligence operatives in Turkey were vetting the flow of weapons to Syrian rebels to ensure they do not fall into the hands of Al-Qaeda militants.
It cited unnamed US officials and Arab intelligence officials as saying the weapons were being paid for by Turkey, Saudi Arabia and Qatar and taken across the Turkish border by a shadowy opposition network.
Turkey is currently home to 32,750 Syrian refugees, said Unal. It also shelters 12 Syrian generals who defected from the Syrian army to join the rebel ranks, he added.
The BBC, meanwhile, reported that Britain has refused to grant the head of the Syrian Olympic Committee a visa to travel to London for next month's Games.
General Mowaffak Joumaa's application was refused because of his links to the Assad government, even though his name does not appear on a European Union list of Syrian officials banned from travelling to the bloc, the BBC reported.
France called for more defections from the Syrian armed forces after the high-profile flight of a MiG fighter pilot to neighbouring Jordan on Thursday.
"Yesterday's defection leads us to call on members of the Syrian army and security forces to continue these defections, these desertions, and no longer to obey the Damascus regime's criminal orders," foreign ministry spokesman Bernard Valero told journalists.
Jordan granted Colonel Hassan Hammadeh asylum after he made an emergency landing at Mafraq base near the border.