Obama decries 'horror' in Aleppo

US President Barack Obama said on Friday the world is "united in horror" at the fighting in Syria's Aleppo and that President Bashar al-Assad and his allies were responsible for atrocities including reported massacres of civilians.

"Responsibility for this brutality lies in one place alone: with the Assad regime and its allies Russia and Iran. And this blood and these atrocities are on their hands," Obama said at a White House news conference.

Obama also called for an international observer force in Aleppo that would ensure an "orderly evacuation" of civilians.