Mark Lowcock, UN under-secretary-general for humanitarian affairs, on Sunday added that the ceasefire reached in Sweden on Thursday should quickly be "translated into a real change on the ground".
"We've got the good news from Sweden - parties have agreed initial steps to de-escalate the conflict or to try to move things forward," he said.
"People I've listened to - parents of starving children, people who have fled from their homes, sometimes multiple times - they're not seeing yet any tangible benefit."
Clashes shook Yemen's flashpoint city of Hodeidah on Sunday after air attacks and deadly fighting on the outskirts overnight, residents said, despite the UN-brokered ceasefire.
Calling for the end of the bloody seven-year war, Lowcock said the results of the biggest ever survey of food security conducted by his organisation in Yemen showed 250,000 people were living in "the highest level of food insecurity, the catastrophe level".
"Last year, the UN were feeding three million people a month, this year it's eight million, next year it needs to be 12 million, that's a mark of the deterioration of the situation," he said.