At least a million Syrians are going hungry, as fighting and checkpoints prevent aid deliveries, the international Red Cross warned on Monday.
"A conservative estimate is a million people without food," said Simon Eccleshall, crisis management chief at the International Federation of Red Cross and Red Crescent Societies (IFRC).
The IFRC's local member, the Syrian Arab Red Crescent (SARC), is the key player in international operations in Syria, where around one third of the pre-war population of 21 million now relies on aid to survive.
But aid efforts have been hit by fighting, which has claimed the lives of 32 of the SARC's 3,000 volunteers, while multiple checkpoints by both sides raise repeated hurdles.
"The SARC only has access to about 85 percent of the territory in Syria on a regular basis," Eccleshall told reporters.
It is able to provide regular supplies to just half of the six million Syrians driven from their homes by the war but still inside the country.
"There are many areas that have not been supplied for months due to the conflict and suburbs around Damascus for almost a year," IFRC spokesman Benoit Carpentier told AFP.
"Obviously, the worst situation is in besieged areas and areas with severe violence. One should also remember that many have not had any income for more than two years. Female-headed households is another big group facing food insecurity," he added.
Eccleshall said there were grave concerns that with the onset of winter, the number needing aid could spiral. Syria's two largest cities Damascus and Aleppo regularly get snow.
Three million Syrians have also fled to neighbouring countries, escaping a conflict that has killed more than 120,000 people.
IFRC has doubled its Syria aid appeal from 53 million to 106 million Swiss francs (43 million to 86 million euros, $58 million to $117).
It has one third of that sum in its coffers, and will be unable to supply food aid by the end of February if more is not forthcoming.
Aid agencies fear growing "compassion fatigue" after almost three years of war in Syria.
"The main challenge that we have today is that for many people in the world, this situation became 'normal'," said Walter Cotte, the IFRC's under-secretary general.
"We need to really highlight that there is a big humanitarian crisis killing people every day," Cotte told reporters.
With Syria's warring sides expected at internationally-brokered peace talks in Geneva on January 22, aid organisations want the humanitarian crisis to be "front and centre" there, said Eccleshall.