Image caption The Turkish military lost radio contact with the F-4 Phantom while it was flying over Hatay province

Turkey's President, Abdullah Gul, has said the Turkish fighter jet shot down by Syria's air defence forces on Friday may have violated Syrian airspace.

Mr Gul said it was routine for warplanes flying at high speed to cross borders for short distances.

Syria has 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.

Relations between Nato-member Turkey and Syria, once close allies, have deteriorated sharply since the uprising against Syrian President Bashar al-Assad began in March 2011. More than 30,000 Syrian refugees have fled the violence across the border into Turkey.

After a cross-border shooting by Syrian security forces in April that left two refugees dead at a camp near the town of Kilis, Turkey said it would not tolerate any action that it deemed violating its security.

Analysis The Turkish response to the downing of one of its fighter jets by Syrian forces has been strikingly low-key. Official statements have been terse. Turkey is not challenging the official Syrian account of what happened, but nor is it yet accepting it. The Syrian military said the F4 reconnaissance jet was shot down as it flew low and fast towards the city of Latakia, just 1km from the coast. Turkey's President Abdullah Gul suggested that it was routine for fast-flying military jets to stray into other countries' air space, but the Syrian account puts this aircraft deep inside its territory, raising big questions about what it was doing. Had it gone badly off course, or was it on some other mission? There are questions too about why Syria shot the aircraft down, rather than try to ascertain its purpose. Two impressions are left by what we know so far from this incident. First, that Syria's sophisticated, Russian-supplied air defence systems are effective, and Syria is willing to use them. Second, that Turkey is taking great care not to be drawn into a military confrontation with Syria.

'Not ill-intentioned'

On Saturday, President Gul said the Turkish government could not ignore the fact that Syria had shot down a Turkish aircraft.

"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.

"It is routine for jet fighters to sometimes fly in and out over [national] borders... when you consider their speed over the sea," he added.

"These are not ill-intentioned things but happen beyond control due to the jets' speed."

Mr Gul said an investigation would look at whether the plane had been shot down in Turkish airspace, and also revealed that Ankara had been in contact with Damascus despite both countries declaring each other's diplomats unwelcome earlier this year.

"We withdrew our envoy from Syria for security reasons. This does not mean that we have no contacts," he explained.

Deputy Prime Minister Bulent Arinc later said the jet had been on a reconnaissance mission, state television reported.

The BBC's Jonathan Head in Istanbul reports that the Turkish government is treating the loss of the aircraft very seriously, but also with great caution.

Turkish press reaction Commentary by Ugur Ergan in Hurriyet "[Turkey] has been stating that the [Syria] problem cannot be solved by military means. So, it would not be right to expect, on account of this incident, activity that could be defined as war between the two countries. Such a situation would mean getting into a war not only with Syria, but with Russia and China at the same time." Commentary by Ilter Turan in Vatan "Even if it [the plane] violated Syrian airspace, this was not the right step for Syria, which has taken a hostile stance by any measure."

Despite public anger over the suffering of civilians in Syria, Ankara has been very reluctant to consider military intervention, our correspondent says.

It will not rush into a military response to this incident either, he adds.

Diplomatic relations between Turkey and Syria have been downgraded to their lowest possible level without completely breaking them.

But the speed with which the coastguards of the two countries organized a joint search-and-rescue operation for the two missing crew members from the plane suggests there are still avenues of communication between their military forces, our correspondent adds.

'Unidentified target'

The Turkish military said it lost radio contact with the F-4 Phantom at 11:58 (08:58 GMT) on Friday while it was flying over Hatay province, about 90 minutes after it took off from Erhac airbase in the province of Malatya, to the north-west.

Alleged flightpath of downed Turkish F-4 Phantom 1. F-4 Phantom takes off from Erhac airbase, Turkey, at approximately 10:28 local time (07:28 GMT), on 22 June 2. Syria says the jet enters its airspace at 11:40 (08:40 GMT) 3. Turkish military loses contact with the plane at 11:58 (08:58 GMT), while it is over Hatay province 4. Syria says its air defences engaged aircraft about 1km (0.6 miles) from the coast and that it crashed into the sea 10km (6 miles) west of Om al-Tuyour

Later, the Syrian military said an "unidentified air target" had penetrated Syrian airspace from the west at 11:40 local time (08:40 GMT), travelling at very low altitude and at high speed.

It said that in line with the laws prevailing in such cases, Syrian air defences engaged the craft, and scored a direct hit about 1km (0.6 miles) from its coastline.

It burst into flames, and crashed into the sea at a point 10km (6 miles) from the village of Om al-Tuyour, off the coast of Latakia province, well within Syrian territorial waters, the statement added.

Syrian television showed a map charting the aircraft's movements, coming in from over the sea near northern Cyprus.

The military statement said that after it "became clear the target was a Turkish military plane which had entered our airspace", the naval commands of the two countries were in touch, and a joint operation was going on to find the missing crew members.