The leaders of several UN agencies have jointly called for the lifting of a humanitarian blockade in Yemen, which they say is making "an already catastrophic situation far worse".
The Saudi-led military coalition has partially lifted the blockade of Yemen's ports in the wake of international outrage - but restrictions on fuel, medicine and food remain in place at the country's points of entry.
World Food Programme Executive Director David Beasley, UNICEF Executive Director Anthony Lake, and WHO Director-General Dr Tedros Adhanom Ghebreyesus have called for action on what they describe as "the worst humanitarian crisis in the world".
In a joint statement, the three leaders said: "The space and access we need to deliver humanitarian assistance is being choked off, threatening the lives of millions of vulnerable children and families."
Officials estimate that around 20 million people, including more than 11 million children, are in urgent need of humanitarian assistance.
The agencies say an estimated 150,000 malnourished children could die within the coming months if they are not treated, while Yemen is already seeing the spread of Diphtheria and a major cholera outbreak.
While the cholera outbreak is said to be waning, officials warn it is likely to flare up again if the embargo is not lifted.
The UN leaders say: "The clock is ticking and stocks of medical, food and other humanitarian supplies are already running low. The cost of this blockade is being measured in the number of lives that are lost.
"If any of us in our daily lives saw a child whose life was at immediate risk, would we not try to save her? In Yemen we are talking about hundreds of thousands of children, if not more. We have the lifesaving food, medicine and supplies needed to save them, but we must have the access that is currently being denied."
They add: “On behalf of all those whose lives are at imminent risk, we reiterate our appeal to allow humanitarian access in Yemen without further delay.”
A deadly civil war has been waging in Yemen since 2015, with Houthi rebels clashing with forces loyal to the government of Abdrabbuh Mansur Hadi.
The Saudi-led multinational coalition backs Hadi's government, with the coalition itself supported by the United States.
The conflict is estimated to have left more than 5,000 civilians dead, and millions more displaced.