Syria said Friday it shot down a Turkish military plane that entered Syrian air space, and Turkey vowed to "determinedly take necessary steps" in response.

It was the most clear and dramatic escalation in tensions between the two countries, which used to be allies before the Syrian revolt began in March 2011. Turkey has become one of the strongest critics of the Syrian regime's brutal response to the country's uprising.

Late Friday, Syria's state-run news agency, SANA, said the military spotted an "unidentified aerial target" that was flying at a low altitude and at a high speed.

"The Syrian anti-air defences counteracted with anti-aircraft artillery, hitting it directly," SANA said. "The target turned out to be a Turkish military plane that entered Syrian airspace and was dealt with according to laws observed in such cases."

Turkey issued a statement Friday night following a two-hour security meeting led by Turkish Prime Minister Recep Tayyip Erdogan, saying Syrian forces downed the plane and that the two Turkish pilots remain missing.

It said Turkey "will determinedly take necessary steps" in response, without saying what those actions would be.

"Following the evaluation of data provided by our related institutions and the findings of the joint search and rescue efforts with Syria, it is understood that our plane was downed by Syria," the statement said, without providing other details.

Relations between Turkey and Syria were already tense before the downing of the F4 plane on Friday.

Turkey has joined nations such as the U.S. in saying that Syrian President Bashar Assad should step down because of the regime's brutal suppression of the uprising in his country. Turkey also has set up refugee camps on its border for more than 32,000 Syrians who have fled the fighting.

Syria and Turkey have expelled each other's ambassadors and Syria has accused Turkey of supporting Syrian opposition and even allowing Syrian rebels to operate out of Turkish soil. Turkey strongly denies the allegations.

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

Foreign Ministry spokesman Selcuk Unal earlier on Friday rejected allegations that Turkey was sending arms and other equipment to Syrian rebels as baseless. Unal said Turkey was not sending weapons to any of its neighbours, including Syria.

Turkey's military provided no details on the downed plane's mission Friday, but some Turkish TV reports said it was on a reconnaissance flight.

Syria claimed the jet violated its air space over territorial waters, penetrating about one kilometre, but that Syrian vessels joined the search for it, according to Turkey's NTV television. It said Syria forces realized that it was a Turkish jet after firing at it.

Ilter Turan, a professor of political science at Istanbul's Bilgi University, told NTV that Syria's action was clearly "hostile," even if it violated its air space.

"They could have either sent their planes to confront it or force it to land, it is a hostile act by any standard," Turan said.

Turan, however, predicted that Syria will try to avoid escalating tensions further.

Erdogan said the plane went down in the Mediterranean Sea about 13 kilometres away from the Syrian town of Latakia. Four Turkish gunboats and three helicopters were searching for the pilots and wreckage of the plane.

The Turkish military said the plane disappeared from its radar and that radio contact was lost at 11:58 a.m. local time Friday during a mission flight.

Some eyewitnesses in Turkey's seaside area of Hatay province told private NTV television that the plane was flying so low they thought it would "hit the roofs." They said the plane then flew toward the sea.

U.S. Secretary of State Hillary Rodham Clinton earlier this month had warned about a massing of Syrian forces near Aleppo, saying such a deployment could be a "red line" for Syria's northern neighbour Turkey "in terms of their strategic and national interests."