Turkey lost a warplane over the Mediterranean today, but its prime minister said he could not confirm reports that Syria had shot it down and had apologised.
If the Syrians did indeed bring down the Turkish F-4 jet, a new crisis could erupt between two prickly Middle Eastern neighbours already at bitter odds over a 16-month-old revolt against Syrian President Bashar al-Assad.
However, in his first public comments on the warplane's loss, Turkish Prime Minister Tayyip Erdogan took a measured tone, telling reporters he could not say whether the plane had crashed or been shot down.
He said he had no word on the fate of the two airmen.
"The chief of general staff has made the necessary statement about the missing plane. I am not saying it was brought down at the point it fell. It is not possible to say this without knowing the exact facts," Erdogan told a news conference.
The Turkish military earlier said one of its planes was missing. Erdogan said Turkish ships and helicopters were searching for the airmen in cooperation with Syrian vessels.
The prime minister spoke for 15 minute about trips he had just completed to Mexico and Brazil before mentioning the loss of the plane.
Erdogan said he had no firm information on a reported Syrian apology and promised a further statement after a security meeting with his interior and foreign ministers and the chief of general staff later in the evening.
Nato-member Turkey, which had drawn close to Syria before the uprising against Assad, turned against the Syrian leader when he responded violently to pro-democracy protests inspired by popular upheavals elsewhere in the Arab world.
Ankara has previously floated the possibility of setting up some kind of safe haven or humanitarian corridor inside Syria, which would entail military intervention, but has said it would undertake no such action without UN Security Council approval.
Turkey hosts about 32,000 Syrian refugees and allows the rebel Syrian Free Army to operate from its territory.
Russia and China, Assad's strongest backers abroad, have fiercely opposed any outside interference in the Syrian crisis, saying envoy Kofi Annan's peace plan is the only way forward.
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.
Lavrov said the Syrian authorities were ready to withdraw troops from cities "simultaneously" with rebels. A Syrian military pullback and a ceasefire were key elements in Annan's six-point peace plan, most of which remains a dead letter.
Annan hit out at some countries he said had taken national initiatives that risked unleashing "destructive competition".
He told a news conference in Geneva that he wanted states with influence on both sides of the conflict to be involved in the peace process, including Iran, Assad's closest ally.
The UN-Arab League envoy was speaking a week before a planned Syria crisis meeting that is in doubt because of Western objections to the Islamic Republic's participation.
Violence raged on unabated in Syria, which appears to be sliding towards a sectarian-tinged civil war pitting majority Sunni Muslims against Assad's minority Alawite sect.
Rebels killed at least 25 members of a mainly Alawite pro-Assad militia, and in a separate incident troops turned heavy machineguns on opposition demonstrators in the northern city of Aleppo, killing 10, opposition activists said.
"Armed terrorist groups committed a brutal massacre against 25 citizens in Darat Azzah," state TV reported, saying more were missing from the village in Aleppo province.
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.
Assad's foes have accused troops 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 today, 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.
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.