Turkey has conceded that the Turkish fighter shot down by Syrian air defences may have crossed into the country’s airspace at the time.

Turkey’s leaders have warned that they will take “necessary” action in response to the shooting down of one of its warplanes by Syria.

Turkey’s president, Abdullah Gül, said Turkish and Syrian forces were working together to search for the two missing crew of the F-4 aircraft, which was shot down over the Mediterranean on Friday and that any cover-up would not be possible.

“There is no doubt that the necessary steps will be taken.” However, he did not elaborate on what these would be.

Gül said the investigation was focusing on whether the plane was brought down within Turkey’s borders or over that of neighbouring Syria, which the Syrian government claims. “Because the consequences could be quite serious, there will be no clear statement before the details [of the incident] are scrutinised,” Gül said.

While the Syrian government claims that it was within its rights to shoot down the plane, which it said was flying low and fast over Syrian territorial waters, Gül said it was “routine” for jets flying at high speeds to violate other countries’ air spaces for short periods of time.

Following the incident, Syria’s state-run news agency, Sana, said the “unidentified aerial target” had been hit by anti-air defences, “according to laws observed in such cases”.

Turkey has been one of the Syrian regime’s most ardent critics over its brutal domestic crackdown and the incident threatens to add a new international dimension to the 16-month internal revolt against the Syrian president, Bashar al-Assad.

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 the Syrian army, which is already struggling to put down a 16-month-old revolt.

Ankara, 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. Turkey now gives refuge to the rebel Free Syrian army president, Bashar al-Assad on its frontier with Syria.

Comments by the Turkish prime minister, Recep Tayyip Erdoğan, echoed Gul’s and were measured in tone.

“As a result of information obtained from the evaluation of our concerned institutions and from within the joint search and rescue operations with Syria, it is understood that our plane was brought down by Syria,” Erdoğan’s office 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 office said after a two-hour emergency meeting between prime minister, the chief of general staff, the defence, interior and foreign ministers, the head of national intelligence and the commander of the air force.

Turkish media had reported earlier that Syria had apologised for the incident, but Erdoğan made no mention of an apology.

Erdoğan, whose enmity with Assad has assumed a strongly personal nature, gave no hint what action he might contemplate.

A statement by the Syrian military said the Turkish plane was flying low, just 1km off the Syrian coast, when it was hit by anti-aircraft fire. The plane fell in Syrian waters seven miles west of the village of Um al-Touyour.

“The navy 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.

Turkish state television interviewed witnesses on the country’s Mediterranean coast, near the Syrian border, who said they saw two low-flying fighter jets pass overhead in the morning in the direction of Syrian waters but only one return.

On the Turkish news channel NTV, Mesut Casin from Yeditepe university said: “Syria is at fault. It had no right to just shoot down a Turkish plane, especially if it was still over international waters. And even if it went into Syrian airspace, Syria should have sent a warning before shooting. I think this will increase international pressure on Syria.”

Ankara has discussed setting up a humanitarian corridor to protect civilians from attacks by the Syrian military and its proxies. But it admits this would be impossible without some form of military intervention, as well as a mandate from the UN security council.

Russia and China have rejected western intervention in Syria and say the only way forward is UN envoy Kofi Annan’s peace plan.

The situation inside Syria remained bloody, with the country apparently sliding towards a sectarian-tinged civil war pitting majority Sunni Muslims against Assad’s minority Alawite sect. Rebels killed at least 25 members of the mainly Alawite pro-Assad Shabiha militia, and in a separate incident troops turned heavy machine-guns 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 Shabiha militiamen had been killed.

Assad’s foes have accused troops and Shabiha 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.

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