Turkey promised yesterday to do "whatever is necessary" in response to Syria shooting down a Turkish military aircraft.

However, Turkish president Abdullah Gul did not contest an assertion by Damascus that the plane had been in its airspace at the time. The Turkish deputy prime minister, Bulent Arinc, later said the jet was not a warplane but a reconnaissance aircraft

The downing of the aircraft, at a point close to the sea borders of both countries, provided a demonstration of Syria's formidable Russian-supplied air defences; one of the many reasons for Western qualms about any military intervention to halt bloodshed in the country.

Ankara's once-friendly relations with Damascus have turned icy over President Bashar al-Assad's violent crackdown on a 16-month-old revolt, but signals from both sides suggest neither wants a military confrontation over the incident.

"It is not possible to cover over a thing like this, whatever is necessary will be done," Gul said, adding that Ankara had been in contact with Syrian authorities.

He said that it was routine for fast-flying jets to cross borders for a short distance and that an investigation would determine if the F-4 fighter was brought down in Turkish airspace.

Syria said it engaged the aircraft in its airspace "according to the laws that govern such situations", and that it crashed into the Mediterranean Sea. The Turkish and Syrian navies are searching for the two crew members.

The military in Syria said the Turkish aircraft was flying low, just one kilometre off the Syrian coast, when it was shot down. "The navies of the two countries have established contact. Syrian naval vessels are participating along with the Turkish side in the search operation for the missing pilots," it said.

With the second-biggest army in Nato, a force hardened by nearly 30 years of fighting Kurdish rebels, Turkey would be a formidable foe for a Syrian military already struggling to put down a popular uprising and an increasingly potent insurgency.

Turkish Prime Minister Tayyip Erdogan, who has already chaired an emergency security meeting, said in a statement: "Turkey will present its final stance after the incident has been fully brought to light and decisively take the necessary steps."

The Turkish media has been less restrained. "They (the Syrians) will pay the price," said the newspaper Vatan, while the Hurriyet daily said "He (Assad) is playing with fire."

The joint Turkish-Syrian rescue operation sits uneasily with Turkey's hosting of the rebel Free Syrian Army fighting to topple Assad, once a personal friend of Erdogan.

The souring of relations over the past year has provoked concern among Turks that Syria may revive its former support for the Kurdish Workers Party (PKK) insurgents in southeastern Turkey.

"It's possible the Turks were sending jets in the area in response to an apparent escalation of the PKK's activities," said Hilal Khashan, political science professor at the American University of Beirut.

"Turkey may suspect that Syria and Iran are supporting Kurdish rebel activities now as a reaction to Turkish support of the Syrian revolt," he said. However, Khashan said he did not expect a harsh military reaction from Turkey. "It is under a tight leash by the United States. They don't want to start a war tomorrow."

A civil war, or something closely resembling one, is already in full swing in Syria, where fighting or shelling engulfed parts of the cities of Aleppo, Hama, Homs, Deir al-Zor and Douma, according to the Syrian Observatory for Human Rights.

The British-based watchdog also reported fierce clashes and shelling in the town of al-Bab in Aleppo province, where army helicopters were flying overhead. It said rockets and gunfire had killed three people in al-Qusair, a town in Homs province. Two men were killed in an ambush by security forces in Hama.

The Syrian army also shelled the eastern city of Deir al-Zor on Saturday, killing at least 28 people, opposition activists said. A source at a hospital in the city said the victims, who included three women and several children, were mostly civilians killed by shelling on their houses in the Old Airport district

Turkey fears the fighting, much of which pits majority Sunni Muslim dissidents and rebels against Assad's Alawite-dominated security forces, could unleash a flood of refugees over its own border and ignite regional sectarian conflict. It already hosts 32,000 Syrian refugees near the border. The opposition Syrian National Council meets in Istanbul.

Ankara has previously floated the idea of setting up a safe haven inside Syria, which would entail military intervention, but has said it would not take such action without UN Security Council approval.

The Syrian president also formed a new government on Saturday, but kept on the heads of the interior, defence and foreign ministries. The reappointment of Defence Minister Daoud Rajha will quash rumours that Rajha had been assassinated by rebels.