The money is needed to address the "constantly increasing humanitarian needs in Yemen" until the end of 2015, Jens Laerke, spokesman for the UN's humanitarian agency, told reporters in Geneva.
He added that more than "21 million people, or 80 per cent of the population, is now estimated to be in need of some form of humanitarian aid and/or protection."
The appeal came as Yemen's warring sides entered a fifth and likely final day of what appeared to be deadlocked consultations with the UN's special envoy for Yemen, Mauritanian diplomat Ismail Ould Cheikh Ahmed, in Geneva.
"People across the country are struggling to feed themselves and their families and basic services are collapsing in all regions of the country."
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Only USD 200 million of the needed cash has so far been received, the UN said, adding that Saudi Arabia's pledge last month of USD 540 million had yet to materialise.
Yemen has been wracked by conflict between Iran-backed Shiite rebels and troops loyal to exiled President Abedrabbo Mansour Hadi, who fled to Saudi Arabia in February.
More than 2,600 people have been killed since then.