NATO strongly condemned as "unacceptable" Syria's downing of a Turkish jet and expressed "solidarity" with Turkey on Tuesday but stopped short of raising any possibility of military intervention.

"We consider this act to be unacceptable and condemn it in the strongest terms," NATO chief Anders Fogh Rasmussen said after emergency consultations at Brussels headquarters.

The talks, gathering ambassadors of the 28-nation Atlantic alliance, were requested by Turkey under Article 4 of NATO's founding treaty. It enables any member to call for consultations should their territorial integrity, political independence or security be considered under threat.

During the talks, which lasted about 90 minutes, "allies have expressed their strong support and solidarity with Turkey", Rasmussen said. "The security of the alliance is indivisible."

At the meeting Turkey's ambassador to NATO outlined the circumstances of the downing of a Phantom 4 jet Friday while on a training mission over international waters. The two men on board remain missing.

The Turkish ambassador said the fighter plane entered Syrian air space for around five minutes and was shot down some 13 nautical miles off the Syrian coast while above international waters. He also said there could be no disputing these facts, according to a Western diplomat.

After his presentation, the ambassadors one after the other expressed their solidarity with Ankara but not one alluded to a possible military response, a diplomatic source said.

"We continue to follow the situation closely and with great concern," Rasmussen said. But he added: "It's my clear expectation that the situation won't continue to escalate."

"What we've seen is a completely unacceptable act and I would expect Syria to avoid such steps in the future," he also said in response to questions.

It is only the second time since the North Atlantic Treaty Organization was set up in 1949 that consultations have been requested under Article 4, the last time in 2003 also being on a request from Turkey during the Iraq war.

Rasmussen underlined that there was no discussion of Article 5 of NATO's founding treaty, which enables the use of force should one or more of the allies come under attack.

Since the onset of trouble in Syria in March 2011 there has been no talk at NATO of boots on the ground, with Rasmussen recently stating that foreign military intervention was "not the right path" in Syria.