The Russian ambassador to Turkey was shot in the back and killed as he gave a speech at an Ankara art gallery on Monday by an off-duty police officer.

Turkey's President Tayyip Erdogan, in a video message to the nation, cast the attack as an attempt to undermine NATO-member Turkey's relations with Russia. He said he had agreed in a telephone call with Russian President Vladimir Putin to step up cooperation in fighting terrorism.

At a special meeting at the Kremlin, Putin ordered increased security at all Russian missions and said "the bandits" who committed the act would feel retribution.

The assassination of an ambassador marks a dangerous escalation of tension in the region and beyond. Security sources said the police officer was off duty and some witnesses said there was no security scanning machine at the entrance.

The attacker was dressed in black suit and tie and stood alone behind the ambassador as he began his speech at the art exhibition, a person at the scene told Reuters.

"He took out his gun and shot the ambassador from behind. We saw him lying on the floor and then we ran out," said the witness, who asked not to be identified. People took refuge in adjoining rooms as the shooting continued.

A video showed the attacker shouting: "Don't forget Aleppo, don't forget Syria!" and "Allahu Akbar" ("God is greatest") as screams rang out. He paced about and shouted as he held the gun in one hand and waved the other in the air. The gunman was killed by special forces. Three other people were injured.

"We regard this as a terrorist act," said Russian Foreign Ministry spokeswoman Maria Zakharova. "Terrorism will not win and we will fight against it decisively."

Erdogan identified the attacker as 22-year-old Mevlut Mert Altintas, who had worked for Ankara riot police for two and a half years.

Turkish Foreign Minister Mevlut Cavusoglu was due to meet his Russian and Iranian counterparts in Russia on Tuesday to discuss the situation in Syria. Officials said the meeting would still go on.

The US State Department, involved in diplomatic contacts with Russia in an attempt to resolve a refugee crisis unfolding around the city of Aleppo, condemned the attack, as did the United Nations Security Council.