The Syrian Arab Red Crescent has estimated that over 2.5 million peple have been displaced by fighting between the government and opposition militants.
The chief spokesperson of the UN High Commission for Refugees, Melissa Fleming, said that the figure could be much higher.
"The figure they are using is 2.5 million. If anything, they believe it could be more, this is a very conservative estimate," she said.
"So people are moving, really on the run, hiding. They are difficult to count and access."
The UN said last week that up to 4 million Syrians would need aid by the end of the year.