The council can't ensure the Syrian people can meet their basic needs, Pillay said.
A 12-page paper published Wednesday by Pillay's office said at least 240,000 people are trapped by conflict in parts of Syria.
Pillay, the U.N. high commissioner for human rights, said international law demands that all parties to the conflict allow civilians safe passage to secure their needs.
"The Security Council is continuing to fail Syrians by not even managing to agree on measures to ensure the provision of basic necessities to people," she said in a statement.
Security Council divisions along pro- and anti-government lines have prevented it from passing resolutions on the Syrian crisis.
The commissioner's paper said all parties to the conflict have prevented people from moving about freely by creating barricades and checkpoints, which left civilians cut off from food, water, fuel and medical supplies.
Some religious leaders have issued edicts allowing people to eat dogs and cats to survive.