"We condemn this act of violence, whatever its source. Our thoughts and prayers are with him and his family," State Department spokesman John Kirby said. The US spokesman did not suggest who might have been behind the attack, which witnesses said was carried out by a gunman demanding "revenge" for Aleppo.
Russian forces were instrumental in helping government troops recapture the city of Aleppo from rebel forces in Turkey's neighbor Syria last week. Washington has previously warned Moscow that its support for Bashar al-Assad's Syrian regime risks radicalizing moderate rebels and boosting support for extremists.