More than 6,400 Syrian refugees have fled to Jordan's Zaatari camp in the past 24 hours, joining 30,000 others who have arrived since the start of the month, the UN refugee agency said.

"Jordan has experienced a record number of refugees crossing," UNHCR spokeswoman Melissa Fleming told reporters in Geneva, noting that December's influx totalled 16,413. The new arrivals bring the total population of the sprawling camp to some 65,000.