The United Nations has launched the largest appeal in its history - seeking $5bn (£3.2bn; 3.7bn euros) for humanitarian aid to Syria.
The UN estimates more than 10 million Syrians - half the population - will need help by the end of the year.
As many as four million children are in need of urgent humanitarian assistance, the UN children's agency Unicef says.
UN humanitarian officials have admitted they may struggle to raise the record sums they are now asking for.
Governments were criticised for being slow to commit funds to the previous UN target of $1.5bn for the first six months of this year, the BBC's Nick Childs says.
UN officials say most of that money - $1.2bn - has now been committed, he adds.
But, in Geneva on Friday, the UN said it had revised up the amount of funding needed because of the worsening security situation in Syria.
'Masking a human tragedy'
One in three Syrians is now in need of urgent humanitarian assistance, UN Emergency Relief Co-ordinator Valerie Amos said in a news conference on Friday.
"Between January and April, the number of people displaced in Syria more than doubled. These are massive figures, but those figures mask a human tragedy," she added.
The UN expects the number of refugees - currently more than 1.5 million - to leap to nearly 3.5 million by the end of 2013.
Antonio Guterres, the UN High Commissioner for Refugees (UNHCR), told reporters that refugee camps were currently receiving 7,000 new arrivals every day - putting huge pressure on Syria's neighbours.
"Funding Syria's humanitarian needs is a matter not only of generosity but enlightened self-interest," he said.
Lebanon and Jordan, which host about half a million refugees each, have joined the appeal, asking for donations of $450 million (£289m) and $380 million (£245m) respectively to cover the financial costs.
Within Syria itself, UN officials estimate that nearly seven million people will be dependent on aid, having been forced to flee their homes.
Many of those within Syria have already suffered or witnessed appalling violence, lost family members and are living without food, shelter, medical care and schooling.
Unicef is warning of a lost generation of young Syrians.
The $5bn sought by the UN would cover only the most basic needs of people until the end of this year.
Aid workers say that, even if the fighting were to stop tomorrow, Syria and its people would still need years to recover.
Responding to the UN's latest appeal, International Development Secretary Justine Greening said the UK is "ready to play its role" and urged other donors to make contributions.
The UK-based Save the Children charity says "money alone will not be enough", calling for greater access for aid agencies to worst affected areas of Syria as an urgent priority.
Access to Qusair
The UN-led appeal was launched just days after Syrian troops backed by Lebanese Hezbollah militants regained control of the key town of Qusair after weeks of fierce fighting with rebel forces.
Qusair, which lies 10km (six miles) from the Lebanese border, is a major supply route for both sides of the conflict.
Hundreds of injured civilians are reported to be trapped in Qusair with little access to medical supplies, food and water.
However, on Friday Russia agreed to a UN Security Council statement demanding "immediate, safe and unhindered" access to the town after blocking an earlier version of the communique.
Meanwhile, the Syrian Observatory for Human Rights, a UK-based activist group, said pro-Assad forces were "massing in their thousands" in the northern province of Aleppo in a bid to cut off rebel supply routes to Turkey.
This follows the rebel fighters' attempted takeover on Thursday of the UN-monitored crossing in the Golan Heights separating Israeli and Syrian troops.
The Syrian government forces retook the symbolically significant position just hours later, an Israeli military force said.