Two million -- half the total -- are people who have fled their homes because of the intensifying civil war and are now living without basic services, including clean water, sanitation and power.
"But these figures may be an underestimate," U.N. Emergency Relief Coordinator Valerie Amos told a conference in Kuwait, where donors were making pledges in response to a $1.5 billion appeal for war-hit Syrians both inside and outside the country.
According to preliminary assessments by the humanitarian arm of the opposition political body, the Syrian National Coalition, some 3.2 million people need aid in opposition-held areas alone, while the Syrian government states that 3 million people have been displaced, Amos said in an opening address.
Access for humanitarian agencies inside Syria is a delicate and contested issue. Medical charity Medecins Sans Frontieres (MSF) says 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.
The Syrian National Coalition has also said there is a contradiction in the United Nations providing aid in partnership with the government of President Bashar Al-Assad.
But the United Nations insists that half of the food aid for 1.5 million people is being provided in disputed or opposition-controlled areas by its partner organisations, and no funding is given directly to the Syrian government.
"While access to opposition-controlled and disputed areas remains a challenge, assistance is being delivered in all affected governorates," Amos said on Wednesday.
BORDERS AND CHECKPOINTS
On Tuesday John Ging, the operations director for the United Nations Office for the Coordination of Humanitarian Affairs (OCHA), said the Syrian government had not yet agreed to let aid workers cross national borders that are not under its operational control, adding that securing permission was a priority.
"This is particularly urgent in the North, as without it we cannot reach the vast majority who are in need in the opposition-held areas there," Ging said in a statement.
Amos said efforts to reach people were also being hampered by the presence of hundreds of militia-style groups. She cited the example of one aid convoy that was trying to get to the devastated western city of Homs late last year. It needed to pass through 21 checkpoints, but after managing to negotiate 20, it was turned back at the last one.
"Control of areas in towns and cities can shift on a daily basis and humanitarian workers have to negotiate with field commanders on the ground to get access," she emphasised.
People in Syria have become "destitute and desperate", as they struggle to get food, healthcare and education amid battles raging in urban areas and rights abuses perpetrated against civilians, Amos said.
"We all know that this is a crisis that demands a political solution. But in the absence of one, we must do all we can to help the Syrian people. We cannot let them down," she said.
DONORS STEP UP
Despite the significant security, logistical and bureaucratic restrictions on aid operations, Amos said the biggest barrier until now had been a shortage of funds.
In December, the United Nations asked for $1 billion to help Syrian refugees in neighbouring countries -- now numbering more than 700,000 -- and a further $500 million to pay for aid operations inside Syria, for the first six months of 2013.
Before Wednesday’s donor conference, donors had given only nine percent of the amount sought.
But at the end of the conference, United Nations Secretary-General Ban Ki-moon said the meeting had “reached its target” and donors had pledged more than the goal of $1.5 billion.
Three wealthy Arab nations led the way, the host country Kuwait pledging $300 million and Saudia Arabia and United Arab Emirates each promising the same amount.