* People "really on the run" in Syria, UNHCR says
* Fighting in Hassaka driving Syrian Kurds into Iraq (Adds detail, comments)
By Stephanie Nebehay
GENEVA, Nov 13 (Reuters) - At least 2.5 million Syrians are believed to have fled their homes because of civil war, aid groups said on Tuesday, more than double previous estimates.
The figure comes from the Syrian Arab Red Crescent, whose volunteers are on the frontlines of the 20-month conflict, delivering aid supplies and evacuating wounded.
"The figure they are using is 2.5 million. If anything, they believe it could be more, that this is a very conservative estimate," Melissa Fleming, chief spokeswoman of the U.N. High Commissioner for Refugees (UNHCR), told a news briefing.
"So people are moving, people are really on the run, hiding. They are difficult to count and to access," she said.
Aid agencies had previously thought there were around 1.2 million internally displaced Syrians.
Only 5 percent of the 2.5 million are believed to be living in public facilities, including warehouses and schools, said Fleming. The rest are staying with host families, making it more difficult to count them.
In recent days, air strikes on the town of Ras al-Ain near the Turkish border have caused some of the biggest refugee movements of the conflict.
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.
For now, the U.N. World Food Programme (WFP) says its food rations are reaching some 1.5 million. The UNHCR aims to provide assistance to 500,000 in Syria by the end of the year, mainly blankets, clothing, cooking kits and jerry cans, Fleming said.
"Unfortunately the recent deliveries have been very difficult, marred by violence and insecurity also spreading to parts of the country that used to be relatively calm," she said.
A Syrian Arab Red Crescent warehouse in Aleppo was apparently hit by a shell, burning 13,000 blankets, she said. Unknown armed men hijacked a truck carrying 600 blankets on its way to Adra, outside Damascus.
The UNHCR has temporarily withdrawn about half of its 12 staff from north-eastern Hassaka province due to fierce fighting and insecurity, Fleming said.
"We see corresponding movement of populations there, Syrian Kurds for the most part, across the border into Iraq," she said.
More than 407,000 Syrian refugees have registered or await registration in the surrounding region - Lebanon, Turkey, Jordan and Iraq - and more are fleeing every day, according to UNHCR. (Editing by Tom Pfeiffer)