BEIRUT International aid to Syria is not being distributed equally, with government-controlled areas receiving nearly all of it while opposition-held zones get only a tiny share, a medical charity said.
Medecins Sans Frontieres (MSF) said rebel fighters had extended and consolidated their hold on large areas of Syria and it estimated at least a third of the country's population - about seven million people - lived in areas beyond government control.
"The current aid system is unable to address the worsening living conditions facing people inside Syria," said MSF president Dr. Marie-Pierre Allié in a statement late on Tuesday.
The statement said "areas under government control receive nearly all international aid, while opposition-held zones receive only a tiny share". It did not give details on what percentage of aid was being distributed on each side.
More than 60,000 people have been killed in nearly two years of violence, according to the United Nations. Government forces have far superior weaponry to the rebels and have levelled many opposition-held areas with aerial strikes and artillery.
The MSF statement said shelter, blankets, fuel, flour and infant formula were in short supply. Unofficial health services are targeted by government forces, it said.
Aid operations are carried out through U.N. partner organisations, primarily the Syrian Arab Red Crescent, which has close ties to President Bashar al-Assad's government.
Rebels control several border posts with Turkey, open to aid, refugees and journalists. But the United Nations says it has not delivered aid through these routes, saying it will only bring supplies into the country through border crossings which are approved by the government.
MSF said international donors meeting in Kuwait on Wednesday must support cross-border humanitarian operations in opposition-held areas in Syria to tackle the aid crisis.
However, the United Nations says it has reached the vast majority of Syrians and hopes to reach more through partnerships with 45 Syrian aid agencies that have been approved by Damascus. But it has not given details on the agencies or said how many of them will work in opposition areas.
Robert Watkins, a senior U.N. official, said on Tuesday that 49 percent of food aid distributed to 1.5 million people in Syria was delivered to areas which were either under the control of the opposition or in contested areas.
"We are able to go to all parts of the country in cooperation with the Syrian Arab Red Crescent, which is our main distribution channel throughout the country," he said.
"There are however parts of the country where we cannot get access. That is more because of security concerns. The fighting is just so active there that we would be putting our aid workers' lives at risk."
The main Syrian opposition body says there is a contradiction in the United Nations providing aid in partnership with Assad's government.
"Is it logical to provide aid to a regime responsible for destroying cities, bombing hospitals and bakeries and displacing the population, so it can fix the dire situation it had created?" the Syrian Coalition said earlier this month.