Veteran diplomat Andrei Karlov was shot nine times in the back by off-duty Turkish policeman Mevlut Mert Altintas at the art gallery opening of a show of Russian photography on Monday.
The brazen killing stunned Ankara and Moscow, which have rowed repeatedly over the Syria conflict but in recent weeks have begun cooperating closely on the evacuations from war-wrecked Aleppo.
An unprecedented three-way meeting on Syria between the foreign ministers of Turkey, Russia and Iran went ahead in Moscow Tuesday despite the assassination, with the diplomats backing a widening of a truce.
Adding to the jitters, with Turkey already on high alert after a string of deadly attacks, an individual also fired outside the US embassy in Ankara overnight.
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Russian President Vladimir Putin ordered his secret services to boost security at home and abroad, and to step up cooperation with foreign intelligence services.
"I ask you through channels of partnership to strengthen your work with the intelligence agencies of other states", he was quoted as saying by Russian news wires.
Erdogan said in a speech in Istanbul that he and Putin agreed in a phone call after the murder that "our expanding areas of cooperation with Russia, particularly on Syria, will not be hampered by this attack".
Dramatic footage of yesterday's assassination showed Karlov stumble and crash to the ground on his back as Altintas brandished his automatic pistol at terrified onlookers who cowered behind cocktail tables.
The lone gunman shouted "Allahu Akbar" ("God is greatest") and "Don't forget Aleppo", vowing that those responsible for events in Syria would be held accountable.
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