The United Nations refugee agency has said that up to 700,000 refugees may flee Syria by the end of the year.
The UNHCR forecast is almost four times higher than its June prediction.
This is due to spiralling violence in Syria where President Bashar al-Assad has responded to rebel military gains with air strikes and artillery bombardment.
Activists said more than 300 people were killed in Syria yesterday including dozens in a town southeast of Damascus.
Rebels also bombed the military staff headquarters in the capital.
Around 294,000 refugees escaping 18 months of conflict have crossed into neighbouring Jordan, Iraq, Lebanon and Turkey, most of them in the last two months.
The flow of refugees surged in August when Assad deployed jets and helicopters to strike rebel-held towns in the northern provinces of Aleppo and Idlib, sending thousands across the border with Turkey.
Heavy military clashes with rebels in Deraa in the south also drove many Syrians into neighbouring Jordan.
UN aid agencies and humanitarian partners issued a funding appeal on Thursday for $488m (€348m) to try to help meet the needs of the rapidly growing number of refugees from Syria.
Since the uprising against Assad erupted in March last year, more than 30,000 people have been killed, the Syrian Observatory for Human Rights said this week.
It said more than 7,000 of those were soldiers and members of the security forces.
The United Nations says many more have been displaced and that 2.5m people in the country need aid and assistance.