The International Committee of the Red Cross has condemned what it calls the "utter contempt for humanity" shown by all sides in the fighting in Iraq.
The Red Crescent says it is unable to operate safely in Falluja
In an unusually critical statement from its headquarters in Iraq, the ICRC said the conflict was having a devastating impact on the people in Iraq.
It urged all warring parties to comply with international humanitarian law and let aid workers carry out their duties.
One of the last aid agencies in Iraq, World Vision, has said it is leaving.
The ICRC pointed out that its statement came in the week that hostage-takers reportedly killed humanitarian worker Margaret Hassan, and US troops allegedly shot dead a wounded fighter.
"As hostilities continue in Falluja and elsewhere, every day seems to bring news of yet another act of utter contempt for the most basic tenet of humanity," said Pierre Kraehenbuhl, the ICRC's director of operations.
"Like any other armed conflict, this one is subject to limits, and they must be respected at all times," he added.
The Red Cross has issued a statement in which it can barely hide its anger, says the BBC's Imogen Foulkes in Geneva.
Mr Kraehenbuhl pointed out that complying with international humanitarian law was "an obligation, not an option", for all sides of the conflict.
He reminded all sides it was absolutely prohibited to kill anyone not actively taking part in the hostilities; to torture or subject them to inhuman, humiliating and degrading treatment; and to take hostages.
All parties in the conflict must provide adequate medical care for the injured, whether friend or foe, and must do everything possible to help civilians with food, water and health care, he added.
"If these rules or any other applicable rules of international humanitarian law are violated, the persons responsible must be held accountable for their actions," he said.
The ICRC appealed for "everything possible to be done" to allow aid agencies to help the thousands of Iraqis who are suffering.
"Regrettably, recent events have again shown just how difficult it has become for neutral, independent and impartial humanitarian organizations to assist and protect the victims of the conflict in Iraq," he said.
'New face of danger'
The ICRC pulled its international staff out of Iraq following the bombing of its Baghdad office last year.
The Iraqi Red Crescent has tried to bring aid to Falluja, but wants reassurances its workers will be safe from insurgents before they go in.
World Vision Australia's chief executive Tim Costello said the organisation was leaving Iraq because of the deteriorating security situation and the "obscene and tragic death" of Margaret Hassan.
Iraq represented "a whole new face of danger and violence" for aid workers who were being deliberately targeted, he said.
World Vision's Iraqi head of operations, Mohammed Hushiar, was shot dead by unknown gunmen in a crowded cafe in the northern city of Mosul on 29 September.
The organisation has been in Iraq for 18 months, and said it had helped about 600,000 people, by improving schools, hospitals, clinics, and water supplies.