As the search continues for two pilots of a Turkish military jet shot down over the Mediterranean Sea, Syria says the shooting was not an act of aggression and that the aircraft violated its airspace.
While the Turkish foreign ministry is to release a statement on Sunday on the incident, a Syrian foreign ministry spokesman told a Turkish news channel the shooting was an accident.
He added, however, that Syria exercised its "sovereign right" to defend itself after Turkey said the jet may have strayed into its neighbour's airspace.
Earlier on Saturday, the Turkish president, Abdullah Gul, said it is not possible to ignore the fact that Syria shot down one of its fighter jets.
"It is not possible to cover over a thing like this, whatever is necessary will be done," he was quoted as saying by state news agency Anatolia on Saturday.
Gul said it was routine for jets travelling at high speed to cross borders for a short distance. He said an investigation into the incident would look at whether the jet was downed in Turkish airspace.
"It is routine for jet fighters to sometimes fly in and out over [national] borders ... when you consider their speed over the sea," he said.
"These are not ill-intentioned things but [they can] happen beyond control due to the jets' speed."
Gul confirmed that Ankara had been in telephone contact with Damascus and that a joint search operation for the jet and missing pilots was under way.
The remarks came after the Syrian military disclosed that it shot down a Turkish fighter jet "over its territorial waters".
The two neighbours are already at bitter odds over a 16-month-old revolt against Syrian President Bashar al-Assad.
"Our air defences confronted a target that penetrated our air space over our territorial waters pre-afternoon on Friday and shot it down. It turned out to be a Turkish military plane," a statement by the military circulated on state media said.
In-depth coverage of escalating violence across Syria
Ban Ki-moon, UN secretary-general, said through a spokesperson on Saturday that he hoped that Turkey and Syria would exercise restraint over Syria's downing of the F-4 fighter jet.
"The secretary-general is following the situation closely. He hopes this serious incident can be handled with restraint by both sides through diplomatic channels," Ban's spokeperson Martin Nesirky said in an emailed statement.
The jet lost radio contact with its base over the eastern Mediterranean near Syria's Latakia, according to a Turkish army statement.
Turkey had said earlier that it had lost contact with one of its military aircraft off its southeastern coast after it took off from Erhac airport in the eastern province of Malatya.
Friends to foes
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.
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Al Jazeera's Anita McNaught said: "This is a dramatic escalation in tensions between two neighbouring countries. Relations have been bad for many months and the worst was when the Syrian army fired on a Turkish refugee camp holding Syrian refugees.
"At that point, Turkey invoked issues of national sovereignty. It was seen as a bit of overreaction then.
"We are in a completely different territory now."
Russia and China, Assad's strongest backers abroad, have fiercely opposed any outside interference in the Syrian crisis, saying UN and Arab League envoy Kofi Annan's peace plan is the only way forward.
Sergei Lavrov, Russian foreign minister, said after talks with his Syrian counterpart that he had urged Syria to "do a lot more" to implement Annan's 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 opposition fighters.
A Syrian military pullback and a ceasefire were key elements in Annan's six-point peace plan.
Annan hit out at some countries he said had taken national initiatives that risked unleashing "destructive competition".