About one million people inside Syria are going hungry due to the difficulty of getting supplies into conflict zones, the UN has said.
The UN's World Food Programme (WFP) is handing out rations to about 1.5 million people in Syria each month, still
short of the 2.5 million deemed to be in need, Elisabeth Byrs, WFP spokeswoman, said on Wednesday.
Bread and fuel particularly are in short supply as the few government-approved aid agencies are stretched to the limit.
"Our main partner, the (Syrian Arab) Red Crescent, is overstretched and has no more capacity to expand further," Byrs told a news briefing in Geneva.
Long queues for bread are now normal in many parts of Syria and there are shortages of wheat flour in most parts of the country due to damage to mills, most of which are located in the embattled Aleppo area, she said.
Only a handful of aid agencies are authorised to distribute relief goods, some of which lack staff, fuel or other material.
Deteriorating security conditions forced the WFP to withdraw its staff from the towns of Homs, Aleppo, Tartous and Qamisly, Byrs said.
Urgent humanitarian aid
The UN last month appealed for $1.5bn to help the millions of Syrians suffering from what it called a dramatically deteriorating humanitarian situation.
In-depth coverage of escalating violence across Syria
Four million people in the country need urgent humanitarian aid, including an estimated two million displaced from their homes mainly by government bombardments of opposition-held areas.
According to UN High Commissioner for Human Rights about 60,000 people have been killed during 21 months of conflict between the forces of President Bashar al-Assad and rebels trying to topple him.
Meanwhile, Syrian refugees frustrated over poor living conditions at a camp in neighbouring Jordan have rioted as humanitarian workers were distributing aid, reports say.
Scuffles broke out at the Zaatari camp early on Tuesday morning after heavy rains and cold weather worsened the plight of refugees there.
"The incident started when a large number of refugees were queueing as usual for food handouts," Al Jazeera's Nisreen El-Shamayleh, reporting from Amman, the Jordanian capital, said.
She added that the refugees were frustrated about the cold weather and rains that flooded some tents and blew away others.
The WFP has said it is facing difficulties in providing food assistance and aid to all internally displaced Syrians, due to access problems, rather than shortage of supply, reported El-Shamayleh.
"The World Food Programme has had problems accessing its shipments in the port of Tartous," she said on how the UN programme abilities are being limited.
"The only local partner it has is the Syrian Arab Red-Crescent to deliver the aid and even that group has also been targeted and has limited capabilites at this time."
The WFP had pulled out non-essential staff from its offices accross Syria last November.
Meanwhile, Palestinian factions in Damascus supporting the Syrian regime have called for a ceasefire after a month of fighting between government troops and rebels in the Yarmouk district in the south of the capital.
In a statement on Tuesday, representatives of Damascus-based Palestinian groups called for a halt to all military operations in order to enable medical teams and food supply trucks to aid civilians affected by the conflict in Yarmouk.
Yarmouk has been home to Palestinian refugees for decades. Many Syrians also live in the area. Since the uprising against Syrian President Assad began in 2011, residents have been divided with some taking up arms against the regime and some fighting to support it.
Palestinians in Syria must remain "neutral" in the country's spiralling crisis, said the statement, which was signed by the Popular Front for the Liberation of Palestine-General Command, the Democratic Front for the Liberation of Palestine and the Nidal Front.
In late December, a PLO official said more than 700 Palestinians have been killed in Syria's violence.
An anti-regime activist in Yarmouk told Al Jazeera that Tuesday's announcement had made no difference on the ground, as clashes and shelling by government forces continued.