UN humanitarian chief Valerie Amos made a new demand Tuesday that the UN Security Council press the Syrian government to allow greater access to trapped civilians, diplomats said.
The UN estimates that more than 2.5 million people are in areas that the UN and other aid agencies cannot reach because of the intensity of the 32-month-old civil war.
"The brutality of this conflict is unacceptable. Even war has rules. In this conflict, the rules are not being respected," Amos was quoted as telling a closed meeting.
"Holding civilians hostage to the conflict is not acceptable," she added, according to diplomats at the meeting.
"The government must be convinced to allow humanitarian access," Amos added.
Amos demanded measures including that the Syrian government lift obstacles to issuing visas for international aid workers.
The UN Security Council agreed on a statement in October calling for greater humanitarian access. Western nations say the Syrian government has largely ignored the call while Russia, President Bashar al-Assad's key ally, says the situation has improved.
The United Nations estimates that about 6.8 million people in Syria need assistance with food, medical supplies and other necessities.
Thousands are fleeing the country each day and the UN predicts that the number of refugees in countries around Syria will rise above three million by the end of the year.