The number of Syrians displaced within their homeland by the vicious civil war has reached 4.25 million, the UN humanitarian office said Tuesday.
"Movement by internally displaced persons continues to be large-scale and fluid, as many Syrians are displaced multiple times," said Jens Laerke, spokesman for the Office of the Coordination of Humanitarian Affairs.
"Over the past months, the number has more than doubled, from an estimated two million to 4.25 million people," he told reporters.
The figures -- combined with more than 1.4 million Syrians who have fled abroad -- mean that more than a quarter of Syria's pre-war population of 22.5 million have been forced to quit their homes since the conflict erupted in March 2011.
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The highest concentration of internally-displaced Syrians is in the flashpoint northern city of Aleppo, where the figure is 1.25 million, followed by the rural area around the capital Damascus, with 705,200, Laerke said.
Laerke said that despite the risks, aid agencies were still managing to cross the often fluid front-lines to reach those in need.
"Overall, from January to April, some 764,000 people have been reached through UN-led relief convoys to hard-to-access areas," he said.
"Out of 10 convoys which crossed conflict lines, five targeted opposition-controlled areas, while another five went to contested areas."
The United Nations has urged Syria to allow the delivery of aid via Turkey to people in dire need in the north of the country.
"So far, however, the Syrian government has said no to cross-border operations," Laerke said.