Turkey has called a Nato meeting after Syrian forces shot down one of its fighter jets, claiming the plane entered its airspace.
Ankara invoked article four of the Nato charter which allows countries to call a meeting if they feel their security is threatened. The consultation is expected to take place in Brussels on Tuesday.
As Syrian and Turkish navies work together to search for the two crew members on board the F-4 Phantom, Foreign Secretary William Hague said he was "gravely concerned" by the "outrageous" incident.
"When I spoke to Turkish Foreign Minister Davutoglu on 23 June, he told me that the plane had been shot down without warning," he added.
"This outrageous act underlines how far beyond accepted behaviour the Syrian regime has put itself and I condemn it wholeheartedly.
"My thoughts and sympathies are with the families and friends of the missing Turkish pilots. I have made clear to foreign minister Davutoglu the UK's strong support for the Turkish government at this difficult time.
Syrian forces said it shot the plane down after the "unidentified" aircraft was seen flying at high speed and low altitude in Syrian airspace.
It said it had shot the plane down about 1 kilometre (0.62 mile), from its coastline, and that the situation had been dealt with "according to laws observed in such cases."
The plane then went down in the Mediterranean Sea about 13km away from the Syrian town of Latakia, Turkey said.
However Turkey's foreign minister insisted that the jet had been shot down in international airspace, and had not been on a reconnaissance mission, despite being a F-4 Phantom, an aircraft commonly used on spying operations.
In an interview with TRT television, Ahmed Davutoglu said that the plane had "momentarily" crossed into Syrian territory but had already returned to international airspace when it was taken down.
His words echo the Turkish president's cautious response on Saturday. Abdullah Gul admitted that the Turkish warplane may have crossed the Syrian border but that it was routine for fighter jets to do so during high speed exercises.
"These are not ill-intentioned things but happen beyond control due to the jets' speed," he told the nation's state news agency, Anatolia.
UN secretary-general Ban Ki-moon has urged for Turkey and Syria to exercise diplomatic restraint in the wake of the incident.
Yesterday Turkey indicated it would respond "decisively" to the attack once the facts had been established, with whatever action was "necessary".
Tension between the neighbouring countries has been escalating since the uprising against Syria's president Bashar al-Assad, with Turkey joining other Nato members in condemning attacks made by brutal regime-led forces. More than 30,000 Syrians have sought refuge in Turkey since violence broke out in Syria more than 16 months ago.
In April Syrian security forces killed two civilians who had fled to Turkey. Following the cross border shooting, Turkey said it would not tolerate any actions that were judged to violate its security.