Nearly seven weeks of Saudi-led air strikes against Iran-backed rebels has caused severe shortages in water, fuel and medical aid as well as heavily disrupting trade, crippling a country dependent on imported food.
"The situation in Yemen is very serious and at the moment the country lacks everything," Dominique Burgeon, Emergencies Director at the Rome-based Food and Agriculture Organization (FAO), told AFP.
"Yemen imports 90 percent of its food needs, as much as 95 percent for some products such as wheat," he said, but "imports are virtually at a standstill."
A lack of fuel means those products which are available cannot be distributed. The agricultural sector has been hit hard, with irrigation systems destroyed and seeds in short supply and livestock are succumbing to diseases.
Despite threats to the ceasefire from rockets shot into border areas of Saudi Arabia from Yemen's rebel-held north, agencies such as the World Food Programme (WFP) and Doctors Without Borders (MSF) were rushing into action.
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But Burgeon said aid alone would not be enough: food provided by the United Nation's WFP, for example, would not reach more than 2.5 million people in a population of more than 24 million.
"It is essential that trade and imports resume," he said, stressing the need for "funds from the international community" to that end.
"Some months ago, 12 million Yemenis were suffering from food insecurity. That number has now gone up to between 14 and 15 million," he said.
"When famine is declared it is already too late for many people. It is absolutely important to act now because the situation is catastrophic," he added.
The five-day humanitarian pause -- which began at 11:00 pm (2000 GMT) on Tuesday -- is the first break in the air war in support of exiled Yemeni President Abedrabbo Mansour Hadi since its launch on March 26.
More than 1,500 people have been killed in the air campaign and fighting between rebel forces and Hadi loyalists, according to the UN.
Saudi Arabia has accused regional rival Iran of arming and funding the rebels, a charge Tehran denies.