Up to 700,000 Syrian refugees may flee abroad by the end of the year, the UN refugee agency said yesterday, almost four times its previous forecast for the exodus from the deepening crisis.

About 294,000 refugees fleeing 18 months of conflict in Syria have already crossed into four neighbouring countries – Jordan, Iraq, Lebanon and Turkey – or are awaiting registration there, the agency said.

Panos Moumtzis, regional refugee co-ordinator for the UN High Commissioner for Refugees (UNHCR), said: "This is a significant outflow taking place – 100,000 people in August, 60,000 in September, and at the moment 2000 or 3000 per day or night.

"For example, Zaatari camp, which is a town, did not exist until 30 days ago, it was a piece of desert, basically."

The UNHCR's previous forecast – made in June – of 185,000 refugees was surpassed in August.

The uprising against President Bashar al Assad, which erupted in March last year, has become an armed insurgency in which activists say 30,000 people have been killed.

Bombardments and heavier fighting between government forces and militants is forcing Syrians to flee, Mr Moumtzis said. Many cross borders with only the clothes on their backs after a perilous journey through frontlines.

He added: "These are families, women and children who escaped Homs, Hama, Deraa, Idlib, Aleppo, who have come out with traumatic experiences and have told us they have fled.

"We have been overwhelmed with catching up basically, because every week we have another 15,000 to 20,000 who have arrived."

Other Syrians who fled the uprising early now find they have nothing left and are registering as refugees.

UN aid agencies and humanitarian partners issued a funding appeal for £300 million to try to meet growing needs before winter sets in.

They are struggling to provide tents suitable for winter, caravans, blankets, clothing, and food for up to 700,000 refugees expected to be staying in camps or as guests in host communities.