Syria confirmed on Saturday it had shot down a Turkish warplane over its territory, sparking a fresh crisis between the two countries, while accusing rebels of having massacred 25 of its supporters.
UN-Arab League envoy Kofi Annan urged world powers to increase pressure on both sides in the Syrian conflict, which monitors say has cost more than 15 000 lives since March 2011.
And UN aid officials said 1.5 million Syrians now needed their help, warning that the situation was still deteriorating.
A Syrian military spokesperson told the official news agency SANA that anti-aircraft batteries opened fire on an "unidentified target" after it entered their airspace, bringing it down in Syrian waters.
They subsequently established it was a Turkish fighter and the two countries' navies were now co-operating in an operation to find the two missing pilots, the agency reported.
A short time earlier, Turkish Prime Minister Recep Tayyip Erdogan issued a written statement saying Syria had shot down a Turkish fighter jet reported missing over the eastern Mediterranean on Friday.
The statement came after an emergency meeting with military and intelligence chiefs and key ministers.
"Turkey will announce its final position and take necessary steps with determination after the incident is entirely clarified," Erdogan said.
A spokesperson for the UN Secretary General Ban Ki-moon said he was following the situation closely.
"He hopes this serious incident can be handled with restraint by both sides through diplomatic channels," said Martin Nesirky.
The latest crisis will further test relations between the two neighbors, already strained over Erdogan's outspoken condemnation of Syria's bloody crackdown on anti-government protests.
In the reported massacre, SANA, citing official provincial sources, said "armed terrorist groups... kidnapped a number of citizens in Daret Azzeh area in the countryside of Aleppo".
It added: "The terrorist groups... committed a brutal massacre against the citizens... shooting them dead and then mutilating their bodies."
It said more than 25 of those kidnapped had been killed, with the fate of others abducted as yet unknown.
The Syrian Observatory for Human Rights said at least 26 government supporters — most of them members of the feared shabiha militia — had been killed.
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 96 people had been killed in violence across the country on Friday, including 50 civilians.
In one incident, 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.
Amateur video distributed by the Observatory showed a group of men carrying the body of a demonstrator through the streets of Aleppo, his torso covered in blood.
And 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.
The bombardment scuppered a new Red Cross attempt to evacuate trapped civilians.
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 12 April, the Observatory said.
In Geneva, Annan called on the international community to do more to end the conflict.
"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 told reporters.
His call came as the UN Office for the Coordination of Humanitarian Affairs said up to 1.5 million Syrians now need humanitarian aid — up from the figure of a million it 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 in the Russian city of Saint Petersburg.
Lavrov urged 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.
"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.