Washington, Dec 20  The US has condemned the assassination of Russian Ambassador to Turkey, Andrey Karlov, in Ankara.

"This heinous attack on a member of the diplomatic corps is unacceptable, and we stand united with Russia and Turkey in our determination to confront terrorism in all of its forms," said US National Security Council Spokesperson Ned Price in a statement.

Karlov was shot and killed on Monday at an art exhibition in the Turkish capital of Ankara, Xinhua news agency reported.

The gunman, who shouted "Don't forget Aleppo," was identified as a Turkish off-duty police officer, according to Turkish security sources.

Karlov was shot in the back while making a speech at a painting exhibition, the Russian embassy said.

Three others wounded were taken to Guven Hospital in central Ankara.

The attack came one day before Turkish Foreign Minister Mevlut Cavusoglu heads to Moscow for talks on Syria with his Russian and Iranian counterparts.