Ahmadiya Juaidi's eyes are wide as she drinks a nutrition shake from a large orange mug, her thin fingers grasping the handle.  

Three weeks ago the 13-year-old weighed just nine kilograms when she was admitted to al-Sabeen hospital in Yemen's capital Sanaa with malnutrition that sickened her for at least the past four years. Now she weighs 15 kilograms.

They are among some 16 million Yemenis - more than half the population of the Arabian Peninsula country - that the United Nations says are going hungry. Of those, five million are on the brink of famine, UN aid chief Mark Lowcock warns.

On Monday the United Nations hopes to raise some $3.85 billion at a virtual pledging event to avert what Lowcock says would be a large-scale "man-made" famine, the worst the world will have seen for decades.

More than six years of war in Yemen - widely seen as a proxy conflict between Saudi Arabia and Iran - have sent the impoverished country spiraling into what the UN describes as the world's largest humanitarian crisis.

Some 80% of Yemenis need help, according to UN data. Twelve aid groups, including Oxfam, Save the Children and Care International, have warned that 2.3 million children under the age of five in Yemen will go hungry this year if governments do not step up their funding on Monday.

In 2018 and 2019, the UN prevented famine due to a well-funded aid appeal, which included large donations from Gulf nations. In 2020 the UN only received just over half the $3.4 billion it needed, which Lowcock said was largely due to smaller contributions from Gulf countries. He urged them to pledge generously for 2021 and pay quickly.