The situation in Syria is "catastrophic," the president of the International Committee of the Red Cross warned on Monday, urging "actions that translate into greater field access" for aid.
The United Nations Children's Fund (UNICEF) and World Health Organisation (WHO) meanwhile condemned the interruption of polio vaccinations in Raqa province, where rebels and jihadists are fighting.
In a statement after a three-day visit to Syria, ICRC president Peter Maurer said most of the country's population was being affected by the conflict.
"It is beyond dispute that the humanitarian situation in Syria is catastrophic," he said.
"I am deeply concerned by the fact that most of the population is directly or indirectly affected by spiralling violence and by constraints on humanitarian aid."
He warned that food and medical supplies were "dangerously short," especially in areas under siege.
Maurer said his visit had focused on urging officials to allow greater access for humanitarian aid.
"I welcome statements made by the various officials I met promising to allow more impartial aid, including medical aid, to everyone affected by the conflict," he said.
"However, with violence rising and needs growing, particularly in besieged areas, actions that translate into greater field access for the ICRC and the Syrian Arab Red Crescent must now follow."
The World Food Programme has said that half of Syria's population is food insecure, and nearly a third requires urgent assistance.
Millions of Syrians are displaced inside the country, and thousands are trapped in months-long sieges without access to food or medicine.
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Among the areas worst affected is the Palestinian Yarmuk camp in southern Damascus, which is controlled by the armed opposition and under a regime siege.
The Syrian Observatory for Human Rights says 41 people have died in the camp since September from hunger or lack of medical care.
Palestinian officials told AFP on Monday that a sixth attempt to deliver aid to the camp was abandoned after the convoy came under fire, without saying who had fired on the vehicles.
With the spread of the conflict that began in March 2011, Syria's basic services have broken down and diseases including polio have spread.
Seventeen cases of polio have been reported, prompting a massive immunisation campaign, but the WHO and UNICEF said Monday that the campaign had been stopped by fighting in northern Raqa province.
The agencies said they "strongly condemn the interruption of a polio immunisation campaign in Al-Raqa Syrian governorate due to the intense fight among fighting groups."
"Polio is a serious risk to children in Syria and the region. It causes permanent disability and all children have the right to be protected against this disease," the groups said.
Raqa has been the scene of violent clashes between fighters from the jihadist Islamic State of Iraq and the Levant and other opposition forces in recent weeks.
The clashes in Raqa and elsewhere in rebel-controlled areas have killed hundreds and brought life to a standstill for civilians.
UNICEF and the WHO said the fighting had not only interrupted immunisation "but is also increasing the suffering of the Syrian people, not least women and children."
"We call on all parties to cease fighting immediately and permit completion of the life-saving polio campaign and delivery of other life-saving humanitarian interventions," they said.