Foreign Secretary William Hague has condemned Syria for shooting down of a Turkish fighter jet.
Mr Hague said the "outrageous" act underlined the need for Bashar Assad's regime to go.
"I am gravely concerned by the Syrian regime's action in shooting down a Turkish military plane on June 22," Mr Hague said in a statement.
"When I spoke to Turkish Foreign Minister (Ahmet) Davutoglu on 23 June, he told me that the plane had been shot down without warning.
"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.
"The Assad regime should not make the mistake of believing that it can act with impunity. It will be held to account for its behaviour. The UK stands ready to pursue robust action at the United Nations Security Council.
"This deplorable incident underlines the urgent need to find a solution to the current crisis in Syria in order to bring an end to the violence and to achieve a genuine political transition. We support the Joint UN and Arab League Special Envoy's continuing efforts on this."