"The figure they are using is 2.5 million," Melissa Fleming, chief spokeswoman of the UN High Commissioner for Refugees (UNHCR), told a news briefing in Geneva.
"If anything, they believe it could be more, this is a very conservative estimate.
"So people are moving, really on the run, hiding. They are difficult to count and access."
The United Nations said on Friday that up to 4 million people inside Syria will need humanitarian aid by early next year when the country is in the grip of winter, up from 2.5 million now whose needs are not fully met.
The UNHCR has temporarily withdrawn about half of its 12 staff from north-eastern Hassaka province due to fierce fighting and insecurity that has resulted in the loss of some aid supplies and driven more Syrian Kurds into Iraq, Ms Fleming said.
