Clashes have been reported in and around the northern Syrian city of Aleppo despite promises by Russia to halt military action to allow in aid.
Russian forces had announced a daily three-hour suspension of military action.
However UN officials said that was too short to take in enough to help the large numbers of people in need.
Aleppo has seen intense fighting recently between rebels and Russian-backed Syrian government forces.
There are also reports of a toxic gas attack on a rebel-held area. Four people died and many others were injured, medical staff say.
The gas was thought to have been chlorine dropped in a barrel bomb, said the Syrian Civil Defence – volunteer emergency response workers.
Elsewhere, Russian planes attacked targets in the northern city of Raqqa, a stronghold of so-called Islamic State.
The city’s water supply had been cut off as a result, the group’s media arm, Amaq, said. The report was confirmed by an anti-IS group.