Aid convoys have reached some besieged areas of Syria where hundreds of thousands of civilians are trapped.
The Russia-US-brokered ceasefire which took effect across Syria from Monday night appears largely to be holding.
The main body representing Syrian opposition groups has set out its plan to end the war under which President al-Assad would step down after six months of talks.
It has long been suspected and now an official investigation reportedly concludes that government troops were behind two toxic gas attacks in Syria.
The Pentagon says US-led coalition aircraft have been scrambled in Syria to protect American special ground forces from attack by government jets.
Striking footage has emerged of children being pulled from the rubble of a building flattened by air strikes in Syria.
There has been heavy fighting in Aleppo as Syrian government forces and rebels battle for control of the city.
The United Nations says it is talking to Russia about getting a longer halt to hostilities, to help the two million people trapped in the divided Syrian city of Aleppo.