Turkey has accused Syria of shooting down a military reconnaissance jet in international airspace without warning and called a NATO meeting to discuss a response to Syrian President Bashar al-Assad.
Amid growing acrimony between the once-friendly neighbours, Syria said its forces had shot dead "terrorists" infiltrating its territory from Turkey, which along with Western and Arab nations has backed the cause of Syrians fighting Assad.
Turkish Foreign Minister Ahmet Davutoglu said the search for two missing pilots was still under way, in coordination with the Syrian authorities. He denied it was a "joint" operation.
He said the RF-4 Phantom jet had been clearly marked as Turkish and dismissed Syria's assertion it had not identified the aircraft before opening fire.
Davutoglu said he also planned to set out Turkey's case at the United Nations Security Council where Western powers are seeking, against Russian and Chinese opposition, to push through a motion that could allow stronger measures against Assad.
Moscow fears this might lead to military action that could undermine its interests in Syria.
In Washington, Secretary of State Hillary Clinton strongly condemned Syria's "brazen and unacceptable act" and vowed close U.S. cooperation with Turkey as part of "our broader efforts to promote a democratic transition in Syria."
The shooting down of the jet "is yet another reflection of the Syrian authorities' callous disregard for international norms, human life, and peace and security," she said in a statement following a conversation on Saturday with Davutoglu.
What began as demonstrations against Assad developed last year into armed rebellion, tipping the country towards a sectarian civil war, with thousands already killed across Syria.
Syrian tanks and artillery shelled the eastern city of Deir al-Zor on Sunday, killing at least 20 people in the second day of bombardment in the oil-producing region, amid heavy fighting with anti-Assad insurgents, opposition activists said.
Davutoglu said the jet was unarmed and had been on a solo mission to test domestic radar systems, but acknowledged it had briefly crossed Syrian airspace 15 minutes before it was hit.
"Our plane was shot at a distance of 13 sea miles from Syria's border in international airspace," Davutoglu said.
"According to the radar images, our plane lost contact with headquarters after it was hit and because the pilot lost control, it crashed into Syrian waters after making abnormal movements," he said. "Throughout this entire period no warning was made to our plane."
Some analysts said the aircraft, in violating Syrian airspace at a time of great sensitivity, could in fact have been testing Syria's Russian-supplied radar and air defences, which would be an obstacle to any possible Western armed action.
Turkey is prickly about its own airspace and that of north Cyprus, where Turkish troops are deployed. Only last month it scrambled jets to intercept an Israeli plane it said had violated northern Cypriot airspace and demanded an explanation.
The foreign ministry said Turkey knew the coordinates of its jet's wreckage, 1,300 metres underwater, but had not found it.
Syria, formally at war with Israel and the target of Israeli air raids in the past, has said the plane was flying fast and low, just one kilometre off its coast when it was shot down as an unidentified intruder. It was only later found to be Turkish.
"TERRORIST" INFILTRATORS
Turkey shelters the rebel Free Syria Army (FSA) and hosts 32,000 Syrian refugees on its southeastern border with Syria, some 50 km (30 miles) from where the Turkish aircraft was shot down. But it denies providing arms for the insurgents.
Syria's state news agency said Syrian forces had confronted "terrorists" who had crossed the Turkish border into the coastal province of Latakia and killed several of them on Sunday.
Syria and Turkey share a border 900 km (550 miles) long.
As if to underline its military reach, Turkey's military announced it had carried out air strikes against nine Kurdish militant targets in northern Iraq in the past three days.
Turkey has often bombed Kurdish fighters seeking more autonomy in southeast Turkey and has even sent ground forces across the border to attack their bases in Iraq.
Around 8,000 Syrian Kurdish refugees have crossed into Iraqi Kurdistan, fleeing the conflict at home, Masoud Barzani, president of the autonomous region, told a news conference.
Turkey's hostility to Assad has mounted steadily since the Syrian leader ignored its advice to enact democratic reform in response to protests that erupted 16 months ago as part of a wider Arab awakening, rather than violently suppressing them.
Assad's security state, dominated by his minority Alawite sect, is now embroiled in a bloody fight for survival against rebels mostly from Syria's Sunni Muslim majority.
NATO envoys will meet on Tuesday at Turkey's request under Article 4 of the military alliance's founding treaty, which provides for states to "consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the parties is threatened".
It stops short of the explicit mention of possible armed responses cited in Article 5.
Britain's foreign minister, William Hague, condemned Syria's downing of the Turkish jet as an "outrageous act" and said his country was ready to support robust action against Damascus by the United Nations Security Council.
SYRIA'S ALLIES
Russia and China have previously blocked any firm measures by the council, twice vetoing draft resolutions on Syria.
Moscow, which said on Sunday it had "no doubt" there were attempts at regime change in Syria, is one of Assad's main arms suppliers and a firm opponent of any outside intervention there. It says Western powers would invite an unpredictable spread of violence if they attempted any intervention in the country.
A ship carrying Russian helicopters to Syria, which turned back after its insurance was cut, is expected to resume its journey accompanied by at least one other vessel, Interfax reported, citing a military source.
Italian Foreign Minister Giulio Terzi condemned the shooting down of the Turkish jet on an "inoffensive flight" as "a further extremely serious and unacceptable action by the Assad regime".
European Union foreign ministers will discuss the Syria crisis, as well as Iran and Egypt, in Luxembourg on Monday.
In a telephone conversation with Davutoglu, Iranian Foreign Minister Ali Akbar Salehi, whose government is a staunch backer of Assad, said he hoped both sides would "settle the issue peacefully to maintain regional stability".
Iraq has described the incident as a serious escalation that demonstrated how Syria's conflict could spread beyond its frontiers and open up ethnic and other tensions in the region.
It remains unclear exactly what the Turkish jet had been doing: a reconnaissance mission to support Syrian rebels or to test Syria's air defences are among the theories. Syrian anti-aircraft gunners may also have been jumpy after a defecting Syrian pilot flew his fighter to Jordan the previous day.
While the incident has aggravated tensions and exposed the risks of Syria's crisis spilling over its borders, neither Ankara nor Damascus seems likely to seek a military showdown.
"What all this tells us is that there are a lot of 'fishy' tactics and strategies going on in the region, with numerous players behind many curtains," said Hayat Alvi, lecturer in Middle Eastern studies at the U.S. Naval War College.
"Nonetheless, the idea that it would be in Turkey's and Syria's respective national interests to engage in military conflict with each other is not plausible. Both sides would have too much to lose and very little to gain."
