More than a million people are likely to have fled Syria within the next few weeks, exceeding the UN's "worst-case scenario", according to Oxfam.
Since the conflict began almost two years ago, 937,000 Syrians have sought refuge in neighbouring countries.
With 5,000 leaving Syria every day to escape the civil war between President Bashar al-Assad's forces and rebel fighters, Oxfam expects the UN's "worst-case scenario" of a million refugees by June to be realised faster than forecast.
The international relief agency says the crisis is "spiralling out of control" and that it and other humanitarian organisations are over-stretched and struggling to cope because aid money has not arrived on the ground.
Donors promised $1.5bn at a conference in Kuwait last month, but so far only 20 per cent has arrived.
"The humanitarian crisis is worsening day by day, leaving agencies struggling to provide help that's desperately needed", said Francis Lacasse, Oxfam's Syria crisis response manager.
"Money that was generously promised a month ago is urgently needed now, to allow agencies to continue providing basic services like food, water and shelter, to ever-growing refugee populations."
Oxfam says there are more than 120,000 refugees at Jordan's Zaatari camp, near the Syrian border, which is almost full.
Mr Lacasse said: "This is likely to be a prolonged crisis and agencies and governments need to prepare for the long haul.
"Even if there was an immediate halt to the violence today, there will be massive humanitarian needs that will need to be addressed for months and years to come. There is no quick fix."
26 February 2013
16 February 2013
31 January 2013