The heads of three UN agencies have issued a fresh plea for the Saudi-led military coalition to lift its blockade on Yemen, warning that "untold thousands" would die.
The coalition closed all air, land and sea access to Yemen last week following the interception of a missile fired towards the Saudi capital, saying it had to stem the flow of arms from Iran to its Houthi opponents in the war in Yemen.
Yemen already has 7 million people on the brink of famine, but without the reopening of all ports that number could grow by 3.2 million, the statement said.
"The cost of this blockade is being measured in the number of lives that are lost," David Beasley, Antony Lake and Tedros Adhanom Ghebreyesus, the heads of the World Food Programme, UNICEF and the World Health Organisation, said on Thursday.
"Together, we issue another urgent appeal for the coalition to permit entry of lifesaving supplies to Yemen in response to what is now the worst humanitarian crisis in the world."
Saudi Arabia has since said that aid can go through "liberated ports" but not Houthi-controlled Hodeidah, the conduit for the vast bulk of imports into Yemen.
For months, the UN has warned that the closure of Hodeidah would dramatically escalate the crisis.
At least 1 million children are at risk if a fast-spreading diphtheria outbreak is not stopped in its tracks, and there is also the risk of a renewed flare-up in cholera, which was on the wane after the most explosive outbreak ever recorded - with over 900,000 cases in the past six months.
"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," the joint statement said.