For two weeks, a battle has raged in Aleppo, generating tragic images of injured civilians amid the rubble.
The city — once the country's most populous, and a commercial hub — is a key prize in the civil war. For four years, it has been divided between government and rebel forces and was in effect a military stalemate.
Russia is among the supporters of Syrian president Bashar Assad, while the U.S. supports rebel forces. They were talking to try to find a way to calm the violence in Syria, but the negotiations collapsed this week.