The Syrian regime accused rebels on Friday of carrying out a "brutal massacre" of 25 of its supporters, as neighbouring Turkey held emergency talks after one of its warplanes went down in the Mediterranean..
Monitors said regime forces fired on demonstrators in Syria's second-largest city Aleppo, killing at least eight people, with another killed in the province of the same name.
Meanwhile, UN-Arab League envoy Kofi Annan urged world powers to increase pressure on both sides in the conflict, which monitors say has cost more than 15,000 lives since March 2011.
"It's time for countries of influence to raise the level of pressure on the parties on the ground and to persuade them to stop the killing and start the talking," he said in Geneva.
In neighbouring Turkey, the plane incident triggered an emergency summit of military, intelligence and government officials.
NTV private news channel quoted unnamed military sources as saying the plane crashed in Syrian territorial waters.
Turkish Prime Minister Recep Tayyip Erdogan, whose country's ties with Syria have soured over the anti-regime revolt, later reportedly said Damascus had apologised over the downed plane.
"Syria immediately offered a very serious apology for the incident and admitted it was a mistake," Haberturk daily newspaper quoted Erdogan as saying on a plane bound from Brazil to Turkey.
"At this moment the air force and navy are conducting search and rescue operations in the eastern Mediterranean, and luckily our pilots are alive; we have just lost a plane," the daily quoted him as saying.
Erdogan refrained from unveiling if the plane was shot down by Syrian air defences, speaking at a televised press conference in Ankara upon his return from Brazil.
On the apology, the prime minister said that he did not have "exact information" but the meeting would reveal "if Syria really apologised and if so, why it did."
He had been told the plane was lost close to the Syrian port city of Latakia.
The Syrian Observatory for Human Rights gave a higher toll for the pro-regime losses in the latest reported massacre, saying at least 26 government supporters -- most of them members of the feared shabiha militia -- had been killed.
In Syria's third-largest city Homs, residents held a small protest despite a renewed bombardment by government forces of rebel-held neighbourhoods, activists said.
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The bombardment scuppered a new Red Cross attempt to evacuate trapped civilians as the United Nations said up to 1.5 million people needed aid.
It came after at least 168 were killed in violence in on Thursday, the highest single-day toll since a UN-backed ceasefire was meant to have taken effect on April 12, the Observatory said.
In the reported massacre, "armed terrorist groups ... kidnapped a number of citizens in Daret Azzeh area in the countryside of Aleppo, according to official sources in the province," state SANA news agency said.
"The terrorist groups ... committed a brutal massacre against the citizens ... 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 (President) Bashar al-Assad's regime," the narrator said, without identifying himself.
The Observatory said a total of at least 55 people were killed in violence across the country on Friday.
Meanwhile, the UN Office for the Coordination of Humanitarian Affairs 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.
On the diplomatic front, Russian Foreign Minister Sergei Lavrov met Syrian counterpart Walid al-Muallem, urging Damascus to do more to implement the plan of peace envoy Kofi Annan.
"We called on them (the Syrian regime) to back up their declarations about readiness to implement the Kofi Annan plan with deeds," Lavrov told state television after the meeting Saint Petersburg, Russia.
"They have already done a lot but they can and must do much more."
Lavrov said Muallem had promised him in the name of Assad that the government was ready for a "synchronised" withdrawal of troops from Syrian towns as long as the rebel opposition did the same.
"The minister assured me of this today," he said.