The murder of Russia's ambassador to Turkey will not bring the conflict in Syria any closer to a resolution, Defence Secretary Sir Michael Fallon has said.
Diplomat Andrei Karlov was gunned down at a photography exhibition in Ankara, with the killer named by the Turkish authorities as policeman Mevlut Mert Altintas.
Dramatic images showed suited assassin Altintas brandishing a pistol as he shouted slogans linked to the war in Syria.
Russian forces have supported Syrian president Bashar Assad's troops in the battle to retake rebel-held areas of Aleppo in recent weeks.
The killer, who fired multiple shots, shouted in Turkish: "Don't forget Aleppo! Don't forget Syria!"
He also shouted "Allahu akbar," the Arabic phrase for "God is great".
After shooting the ambassador, the gunman climbed to the second floor of the same building and a 15-minute shoot-out with police ensued before he was killed, Turkey's Anadolu news agency reported.
Foreign Secretary Boris Johnson condemned the "despicable murder" as a "cowardly attack".
And Defence Secretary Sir Michael told MPs it was "a shocking act involving a diplomat who should otherwise, of course, enjoy proper protection", adding: "His murder does not bring any conflict in the Middle East further to a resolution."
Russian President Vladimir Putin said that the killing of Mr Karlov was a "provocation aimed at derailing Russia-Turkey ties and the peace process in Syria".
Turkish President Recep Tayyip Erdogan, in a video message being shown on several Turkish TV channels, said: "This is a provocation to damage the normalisation process of Turkish-Russian relations."
He added: "Both the Russian and Turkish administrations have the determination not to fall for this provocation."
Both leaders said that Russian investigators will be part of the official probe into the assassination.
Richard Moore, the UK's ambassador to Turkey, said on Twitter: "Devastating news about Andrei Karlov. My wife & my thoughts with his wife Marina & family & to all colleagues at Russian Embassy."
He described Mr Karlov as "quietly spoken, hospitable (and) professional".