More than four million people inside Syria are in desperate need of aid, up sharply from 2.5 million in September, the UN's humanitarian agency said Tuesday.
"We are watching a humanitarian tragedy unfold before our eyes," Valerie Amos, the UN Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator, told reporters in Geneva.
A graphic from her office showed how the need for humanitarian aid had spiralled from March last year, when some one million people were listed, to 2.5 million in September and four million by January 2013.
She described the situation in the war-torn country as "devastating."
The United Nations says at least 70,000 people have been killed in the near two-year conflict, while some 2.5 million have been displaced by the fighting but remain in Syria.
Earlier Tuesday, the UN's refugee agency said the number of Syrians who have fled their conflict-ravaged homeland has now topped 850,000.
Amos said more than 250,000 had fled over a two-month period.
Only a year ago, the United Nations said 33,000 Syrians had fled the conflict which erupted in March 2011 as the regime of President Bashar al-Assad launched a bloody crackdown on protests.
The United Nations has warned that refugee numbers could reach 1.1 million within months in what has become an increasingly radicalised civil war in the nation of almost 21 million.
Most of the refugees have fled to neighbouring Lebanon, Jordan, Turkey and Iraq.