Syria's five-year-old war pits Syrian President Bashar al-Assad, backed by Russia, Iran and Arab Shi'ite Muslim militias, against myriad Sunni Muslim rebel groups, including some supported by the United States, Turkey and Gulf Arab states.
The conflict's devastating human toll includes hundreds of thousands of casualties from poison gas attacks, air raids on hospitals, and starvation. Nearly half the country's population of 22 million have been displaced.
A seven-day cease-fire negotiated by the United States and Russia collapsed last week amid violence flared up around the country once again.