Fresh efforts to revive peace talks on Syria are due to begin in Geneva.
UN envoy Staffan de Mistura is to meet delegates of the Syrian government and some rebel groups, as well as regional powers such as Turkey and Iran.
Islamist militant groups such as Islamic State (IS) and al-Nusra Front have not been invited, despite the power they hold on the ground.
The talks come as Amnesty International accuses the government and rebels of war crimes in the city of Aleppo.
The group has published eyewitness testimony of the devastation caused by improvised barrel bombs, which it says have been dropped by government forces on hospitals, mosques and schools.