WASHINGTON -- The United States on Monday 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 U.S. 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.

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 in the Turkish capital, the Russian embassy said.

Three other wounded people 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.