Thousands of civilians are believed to be trapped in Syria's central city of Homs, as government forces continue their heavy bombardment of the city, the UN said.
Both government forces and opposition fighters appeared to be digging in, preventing residents from accessing food and medicine.
UN humanitarian officials say they are providing food to half a million people in Syria, but not in Homs, which means many there are desperate for food, clean water and medical help.
Al Jazeera's Jane Arraf reports.
