According to the UN, 1 million Syrians are going hungry and helpless due to the 22-month civil conflict in the country.
The World Food Program (WFP) says it is helping 1.5 million Syrians, but continued fighting and an inability to use the port of Tartus to deliver food mean many people are not receiving aid.
The UN estimates that more than 60,000 people have been killed in the uprising, which began in March 2011.
Rebels have gained control of swathes of northern Syria in recent months.
The increasingly dangerous situation meant the WFP had pulled its staff out of its offices in Homs, Aleppo, Tartus and Qamisly, said agency spokeswoman Elisabeth Byrs.
Late 2012 saw a sharp rise in the number of attacks on WFP aid trucks, said the agency, which has also been hit by fuel shortages.
Meanwhile, the UN refugee agency said the number of refugees fleeing the violence in Syria had leapt by nearly 100,000 in the past month.
It said there were 597,240 registered refugees and individuals awaiting registration as of January 6 - up from 509,559 the month before.
The UN estimates that around four million Syrians are in need of humanitarian aid.
Opposition forces have been making considerable gains in recent weeks, but their efforts to take control of areas around major cities including Damascus have met with stiff resistance and increasingly destructive air strikes.
On Monday, the New York Times reported that Israeli intelligence indicated Syrian troops were mixing chemicals – suspected to be the deadly nerve gas sarin – at two storage sites, and filling dozens of 500-pound (225 kg) bombs that could be loaded on airplanes.
The reports in late November prompted a flurry of international statements warning the regime of President Bashar al-Assad against using chemical weapons on his own people.