A group of five special envoys to the United Nations working on issues ranging from torture and arbitrary detentions to the right to food said they were alarmed by what they said were incidents of civilian suffering as a method of war.
They said in a statement from Geneva civilians in Syrian were deprived of access to food, water and medical care because of acts of violence.
"These acts are morally abhorrent, and present a major obstacle to building peace," they stated. "We are outraged by the extreme human suffering caused by the apparent blatant disregard for human rights and humanitarian law."
The United Nations estimates there are more than 6.5 million people internally displaced by conflict, 9.3 million are in need of humanitarian aid and 6 million need some form of food assistance.
Robert Mardini, director of Middle East operations for the International Committee of the Red Cross, said in a separate statement that agreements reached during Syrian negotiations in Geneva regarding access to Syrian civilians on the ground weren't enough.
"What is needed now is for the talk to be backed up with action" he said.