"Two years of war have devastated Yemen and millions of children, women and men desperately need our help," warned UN humanitarian aid chief Stephen O'Brien in a statement.
"Without international support, they may face the threat of famine in the course of 2017 and I urge donors to sustain and increase their support to our collective response."
The appeal from UN agencies and other humanitarian organisations aims to gather funds to help some 12 million of the nearly 19 million people expected to need assistance across Yemen this year.
The poor Arab country has been engulfed in war for years, but the conflict escalated dramatically in March 2015 when the Saudi-led coalition launched air raids against Shiite Huthi rebels, who had taken over the capital and seized swathes of the country's centre and north.
Nearly 7,500 people have been killed and more than 40,000 injured since the conflict escalated two years ago, while more than two million people remain displaced inside Yemen, according to UN numbers.
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But the fighting has also dramatically exacerbated the drawn-out humanitarian crisis in one of the world's poorest countries, leaving a full two thirds of the population in need of aid.
More than 10 million people need immediate, life-saving aid, including more than two million children who are acutely malnourished.
Nearly half a million children under five were meanwhile suffering from life-threatening severe, acute malnutrition at the end of 2016 -- a 57-percent increase over 2015, Wednesday's report said.
Last year, UN agencies and other partners provided aid to 5.6 million people in Yemen. This year, they hope to more than double that number.
The country is almost entirely dependent on imports, most of which transit through the Hudaydah port, which was bombed by the coalition in 2015.
And the Saudi-led coalition's shutdown of the Sanaa airport in August 2016 has had a heavy toll on civilians because medicine cannot be flown in and Yemenis cannot receive treatment abroad.