The pace of Syrian refugees arriving in Zaatri camp in northern Jordan has doubled, with 10,200 arriving in the past week.
A spokeswoman for the UN High Commissioner for Refugees said it could be the start of a major influx into Jordan.
Melissa Fleming said the UNHCR has "revised significantly upward" its forecast of 185,000 Syrian refugees registering in Jordan, Iraq, Lebanon and Turkey.
The agency said 200,000 refugees could flee to Turkey if the crisis deepens.
There are now nearly 70,000 Syrian refugees registered in Jordan, although thousands more have not signed up for assistance, according to the UNHCR.
"The refugees say that many thousands more are waiting to cross and violence around Deraa, which is not far across the border, is the reason," Ms Fleming said.
She said that Syrian refugees arriving in Jordan "reported being bombed as they were trying to cross".
"They also reported that there was shelling, mortars and other weapon-fire, pointing to a very treacherous journey to get out," she added.
There has also been a sharp rise in the exodus from Syria to Turkey, where up to 5,000 people have been arriving every day over the past two weeks, she said.
More than 3,000 Syrians had crossed over in the past 24 hours alone.
Turkish authorities have refused to accept more refugees since the weekend because their refugee camps are full.
They said new camps are being built and refugees will be accepted once they are completed.
Around 10,000 Syrians are waiting on the Syrian side of the border as Turkey rushes to build the new camps.
Turkey has so far taken in over 80,000 Syrian refugees.
Turkey's Foreign Minister Ahmet Davutoglu yesterday called for more help from other countries with the refugee crisis and said his country would stress this at a UN Security Council meeting on Syria later this week.
UNHCR spokeswoman Sybella Wilkes said the agency is working with the Turkish government and is ready to send in stocks at short notice.
Elsewhere, 12 people have been killed in a car bombing at a funeral in Damascus.
A car bomb exploded at the entrance to a Druze cemetry in the Jaramana district in southeast Damascus.
One witness said that up to 150 had been injured.
The Syrian Observatory for Human Rights said that the two men being buried were supporters of Assad, who is fighting to crush a 17-month-old uprising against his rule.