The Syrian Arab Red Crescent estimates that 2.5 million people are internally displaced within Syria by civil war, doubling the previous figure of 1.2 million used by aid agencies, the United Nations refugee agency said on Tuesday.

"The figure they are using is 2.5 million. If anything, they believe it could be more, this is a very conservative estimate," Melissa Fleming, chief spokeswoman of the U.N. High Commissioner for Refugees (UNHCR), told a news briefing in Geneva.

"So people are moving, really on the run, hiding. They are difficult to count and access," she said.